diff --git a/package-lock.json b/package-lock.json
index 20480cb..9e9dee2 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -29,6 +29,9 @@
       },
       "engines": {
         "node": ">=14.0.0"
+      },
+      "peerDependencies": {
+        "@fink/js-interop": ">2.1"
       }
     },
     "node_modules/@babel/code-frame": {
@@ -580,9 +583,10 @@
       }
     },
     "node_modules/@fink/js-interop": {
-      "version": "2.0.1",
-      "resolved": "https://registry.npmjs.org/@fink/js-interop/-/js-interop-2.0.1.tgz",
-      "integrity": "sha512-WDjgBWf6hSS3aAP9lMM+OYJOQTOaWoPvWDgN+7IakSacplsX2omk6bdpE1sYBwSLEqfAQ0ZuyybfdGv83WM/WA==",
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/@fink/js-interop/-/js-interop-2.1.0.tgz",
+      "integrity": "sha512-6yxsatYmTb/exBvItoZZYIWv0uIqyuvKW5AaOu/Un94uL5GNRVD6vJYbCX7V1xp4xG+asRTn6uQnzoIn7fTN+A==",
+      "license": "MIT",
       "engines": {
         "node": ">=14.0.0"
       }
@@ -601,9 +605,9 @@
       }
     },
     "node_modules/@fink/loxia": {
-      "version": "18.2.0",
-      "resolved": "https://registry.npmjs.org/@fink/loxia/-/loxia-18.2.0.tgz",
-      "integrity": "sha512-MGdfJwGFOtUqbs23/TMqS6PLWjPdo9P0hIcCZ1E2bkS/r6GKCrIBR8+ytq5cngTVkWbw2u3Nd7Qc4trLKBA0Iw==",
+      "version": "18.3.0",
+      "resolved": "https://registry.npmjs.org/@fink/loxia/-/loxia-18.3.0.tgz",
+      "integrity": "sha512-kiMZ334cYMw7zAJLapWjb9MC3/u+mYz7UD9biD4P9YTdoHLQf+2JSZ4+PuAXjiE24RYrzU2nfZTkZx1kcfCtKg==",
       "dev": true,
       "dependencies": {
         "@babel/core": "^7.10.5",
@@ -18366,9 +18370,9 @@
       }
     },
     "@fink/js-interop": {
-      "version": "2.0.1",
-      "resolved": "https://registry.npmjs.org/@fink/js-interop/-/js-interop-2.0.1.tgz",
-      "integrity": "sha512-WDjgBWf6hSS3aAP9lMM+OYJOQTOaWoPvWDgN+7IakSacplsX2omk6bdpE1sYBwSLEqfAQ0ZuyybfdGv83WM/WA=="
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/@fink/js-interop/-/js-interop-2.1.0.tgz",
+      "integrity": "sha512-6yxsatYmTb/exBvItoZZYIWv0uIqyuvKW5AaOu/Un94uL5GNRVD6vJYbCX7V1xp4xG+asRTn6uQnzoIn7fTN+A=="
     },
     "@fink/larix": {
       "version": "15.2.0",
@@ -18381,9 +18385,9 @@
       }
     },
     "@fink/loxia": {
-      "version": "18.2.0",
-      "resolved": "https://registry.npmjs.org/@fink/loxia/-/loxia-18.2.0.tgz",
-      "integrity": "sha512-MGdfJwGFOtUqbs23/TMqS6PLWjPdo9P0hIcCZ1E2bkS/r6GKCrIBR8+ytq5cngTVkWbw2u3Nd7Qc4trLKBA0Iw==",
+      "version": "18.3.0",
+      "resolved": "https://registry.npmjs.org/@fink/loxia/-/loxia-18.3.0.tgz",
+      "integrity": "sha512-kiMZ334cYMw7zAJLapWjb9MC3/u+mYz7UD9biD4P9YTdoHLQf+2JSZ4+PuAXjiE24RYrzU2nfZTkZx1kcfCtKg==",
       "dev": true,
       "requires": {
         "@babel/core": "^7.10.5",
diff --git a/package.json b/package.json
index 2a12d9c..7eb8d4a 100644
--- a/package.json
+++ b/package.json
@@ -55,6 +55,9 @@
     "npx-run": "^2.1.2",
     "semantic-release": "^17.2.1"
   },
+  "peerDependencies": {
+    "@fink/js-interop": ">2.1"
+  },
   "dependencies": {
     "@babel/core": "^7.10.5",
     "@babel/traverse": "^7.10.5",
diff --git a/src/lang/async/init.test.fnk.snap b/src/lang/async/init.test.fnk.snap
index e60d225..bc7f543 100644
--- a/src/lang/async/init.test.fnk.snap
+++ b/src/lang/async/init.test.fnk.snap
@@ -1,73 +1,39 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`await compiles 1`] = `
-"export const task1 = async (foo) => -(await foo);
+"import { _unfold_ } from \\"@fink/js-interop/runtime.js\\";
+export const task1 = async (foo) => -(await foo);
 export const task2 = async (foo) => await (foo + 4);
 export const task3 = async (foo) => {
   const bar = await foo();
   const spam = await foo();
   return bar + 123;
 };
-export const a_gen = function unfold(ˆinitial_5) {
-  return {
-    async *[Symbol.asyncIterator]() {
-      let ˆitem_1 = ˆinitial_5;
+export const a_gen = _unfold_(async (curr = 0) => {
+  const ˆvalue_1 = shrub;
 
-      while (true) {
-        const curr = ˆitem_1 === undefined ? 0 : ˆitem_1;
-
-        let _do_result;
-
-        ˆmatch_4: {
-          const ˆvalue_3 = shrub;
-
-          if (ˆvalue_3 === spam) {
-            _do_result = await ni(curr);
-            break ˆmatch_4;
-          }
-
-          {
-            _do_result = curr + 1;
-            break ˆmatch_4;
-          }
-        }
-
-        const ˆresult_2 = _do_result;
-        _do_result = undefined;
-        yield ˆresult_2;
-        ˆitem_1 = ˆresult_2;
-      }
-    }
+  if (ˆvalue_1 === spam) {
+    return await ni(curr);
+  }
 
-  };
-};
+  {
+    return curr + 1;
+  }
+}, 1, false, false);
 await ni;"
 `;
 
 exports[`await handles awaiting async iterables 1`] = `
-"let _do_result;
+"import { _unfold_ } from \\"@fink/js-interop/runtime.js\\";
+
+let _do_result;
 
 {
   let _do_result2;
 
   {
     let ˆpipe_result_3 = undefined;
-
-    _do_result2 = ˆpipe_result_3 = function unfold(ˆinitial_6) {
-      return {
-        async *[Symbol.asyncIterator]() {
-          let ˆitem_4 = ˆinitial_6;
-
-          while (true) {
-            const cntr = ˆitem_4 === undefined ? 0 : ˆitem_4;
-            const ˆresult_5 = (await cntr) + 1;
-            yield ˆresult_5;
-            ˆitem_4 = ˆresult_5;
-          }
-        }
-
-      };
-    }(ˆpipe_result_3);
+    _do_result2 = ˆpipe_result_3 = _unfold_(async (cntr = 0) => (await cntr) + 1, 1, false, false)(ˆpipe_result_3);
   }
   const ˆitems_1 = _do_result2;
   _do_result2 = undefined;
diff --git a/src/lang/call/pipe.test.fnk.snap b/src/lang/call/pipe.test.fnk.snap
index 9fe5ee3..5da6c09 100644
--- a/src/lang/call/pipe.test.fnk.snap
+++ b/src/lang/call/pipe.test.fnk.snap
@@ -1,7 +1,8 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`pipe compiles 1`] = `
-"{
+"import { _map_ } from \\"@fink/js-interop/runtime.js\\";
+{
   let ˆpipe_result_1 = undefined;
   ˆpipe_result_1 = foo(ˆpipe_result_1);
 
@@ -16,19 +17,7 @@ exports[`pipe compiles 1`] = `
 }
 {
   let ˆpipe_result_2 = [1, 2, 3];
-
-  ˆpipe_result_2 = function map(ˆitems_5) {
-    return {
-      *[Symbol.iterator]() {
-        for (const ˆitem_3 of ˆitems_5) {
-          const item = ˆitem_3;
-          const ˆresult_4 = item * 2;
-          yield ˆresult_4;
-        }
-      }
-
-    };
-  }(ˆpipe_result_2);
+  ˆpipe_result_2 = _map_((item) => item * 2, 1, false, false)(ˆpipe_result_2);
 }"
 `;
 
diff --git a/src/lang/context.fnk b/src/lang/context.fnk
index 6560e14..4fbaaeb 100644
--- a/src/lang/context.fnk
+++ b/src/lang/context.fnk
@@ -15,7 +15,7 @@ add = fn type, op, transformer: fn {transformers, ...ctx}:
 
 
 
-add_with_builtin = fn type, op, impl_uri, transformer: fn ctx:
+add_with_runtime = fn type, op, transformer, impl_uri: fn ctx:
   built_in_name = '_${op}_'
   pipe ctx:
     add_runtime_fn built_in_name, ?
diff --git a/src/lang/init.fnk b/src/lang/init.fnk
index 46af664..46f80c1 100644
--- a/src/lang/init.fnk
+++ b/src/lang/init.fnk
@@ -17,7 +17,7 @@
 {add_arithmitic} = import './arithmitic/init.fnk'
 {add_block} = import './block/init.fnk'
 {add_partial} = import './partial/init.fnk'
-{init_builtins} = import './runtime.fnk'
+{init_runtimes} = import './runtime.fnk'
 
 
 
@@ -60,7 +60,7 @@ init_ctx = fn code, filename, options:
     ignoreable_imports: []
 
   pipe ctx:
-    init_builtins
+    init_runtimes
     add_transformers
 
 
diff --git a/src/lang/iterable/common.fnk b/src/lang/iterable/common.fnk
index b4265f6..9db3b3d 100644
--- a/src/lang/iterable/common.fnk
+++ b/src/lang/iterable/common.fnk
@@ -13,14 +13,32 @@ babe_types = import '@babel/types'
 transform_with_runtime_lib = fn iter_fn, node, ctx:
   args_len = length node.args
 
-  [use_spread, exprs] = match node.exprs:
-    [..., {type: 'spread'}]:
+  [use_spread, exprs] = match node:
+    {op: not ? in ['map', 'unfold']}:
+      [[], node.exprs]
+    {exprs: [..., {type: 'spread'}]}:
       spread = {type: 'literal', value: 'true', loc: node.loc}
       [...exprs, last] = node.exprs
-      [spread, [...exprs, last.right]]
+      [[spread], [...exprs, last.right]]
     else:
       spread = {type: 'literal', value: 'false', loc: node.loc}
-      [spread, node.exprs]
+      [[spread], node.exprs]
+
+  [is_async, args] = match node.args:
+    [{op: 'await'}]:
+      async = {type: 'literal', value: 'true', loc: node.loc}
+      [{right}, ...rest] = node.args
+      [async, [right, ...rest]]
+    else:
+      async = {type: 'literal', value: 'false', loc: node.loc}
+      [async, node.args]
+
+  [fold_initial, final_args] = match node:
+    {op: 'fold', args: [, {type: 'assign'}]}:
+      [first, {left, right}, ...rest] = args
+      [[right], [first, left, ...rest]]
+    else:
+      [[], args]
 
   [result, end_ctx] = transform
     rec:
@@ -29,13 +47,13 @@ transform_with_runtime_lib = fn iter_fn, node, ctx:
       args: seq:
         rec:
           type: 'fn'
-          args: node.args
+          args: final_args
           exprs: exprs
           loc: node.loc
         {type: 'number', value: '${args_len}', loc: node.loc}
-        {type: 'literal', value: 'false', loc: node.loc}
-        use_spread
-
+        is_async
+        ...use_spread
+        ...fold_initial
       loc: node.loc
     ctx
   [result, end_ctx]
diff --git a/src/lang/iterable/filter.test.fnk b/src/lang/iterable/filter.test.fnk
index 8b8ad3d..04e4b6e 100644
--- a/src/lang/iterable/filter.test.fnk
+++ b/src/lang/iterable/filter.test.fnk
@@ -3,11 +3,10 @@
 
 
 
-describe 'gen 2', fn:
-  it 'compiles', fn:
+describe 'filter', fn:
+  it 'compiles with default runtime', fn:
     expect
       fink2js '
-        {_filter_} = import "@fink/js-interop/iter.js"
         filter item:
           item % 2 == 0
       '
@@ -15,18 +14,35 @@ describe 'gen 2', fn:
 
     expect
       fink2js '
-        {_filter_} = import "@fink/js-interop/iter.js"
         filter item, cntr=0:
           [item < 1 and cntr % 2 == 0, cntr + 1]
       '
       to_match_snapshot
 
 
+  it 'compiles with custom runtime', fn:
+    expect
+      fink2js '
+        {_filter_} = import "./filter.fnk"
+        filter item:
+          item % 2 == 0
+      '
+      to_match_snapshot
 
-describe 'filter', fn:
+    expect
+      fink2js '
+        filter item, cntr=0:
+          [item < 1 and cntr % 2 == 0, cntr + 1]
+      '
+      to_match_snapshot
+
+
+
+describe 'filter legacy', fn:
   it 'compiles', fn:
     expect
       fink2js '
+        {_filter_} = import "@fink/loxia:iter:legacy"
         filter item:
           item % 2 == 0
 
@@ -43,6 +59,7 @@ describe 'filter', fn:
   it 'compiles with accu', fn:
     expect
       fink2js '
+        {_filter_} = import "@fink/loxia:iter:legacy"
         filter item, cntr=0:
           [item < 1 and cntr % 2 == 0, cntr + 1]
       '
@@ -50,6 +67,7 @@ describe 'filter', fn:
 
     expect
       fink2js '
+        {_filter_} = import "@fink/loxia:iter:legacy"
         filter _, is_even=false:
           [is_even, not is_even]
       '
@@ -57,6 +75,7 @@ describe 'filter', fn:
 
     expect
       fink2js '
+        {_filter_} = import "@fink/loxia:iter:legacy"
         filter , is_even=false:
           [is_even, not is_even]
       '
@@ -68,6 +87,7 @@ describe 'filter async', fn:
   it 'compiles', fn:
     expect
       fink2js '
+        {_filter_} = import "@fink/loxia:iter:legacy"
         filter item:
           await item % 2 == 0
       '
diff --git a/src/lang/iterable/filter.test.fnk.snap b/src/lang/iterable/filter.test.fnk.snap
index 240e5fc..e9b45d4 100644
--- a/src/lang/iterable/filter.test.fnk.snap
+++ b/src/lang/iterable/filter.test.fnk.snap
@@ -15,7 +15,31 @@ exports[`filter async compiles 1`] = `
 });"
 `;
 
-exports[`filter compiles 1`] = `
+exports[`filter compiles with custom runtime 1`] = `
+"import { _filter_ } from \\"./filter.js\\";
+
+_filter_((item) => item % 2 === 0, 1, false, false);"
+`;
+
+exports[`filter compiles with custom runtime 2`] = `
+"import { _filter_ } from \\"@fink/js-interop/runtime.js\\";
+
+_filter_((item, cntr = 0) => [item < 1 && cntr % 2 === 0, cntr + 1], 2, false, false);"
+`;
+
+exports[`filter compiles with default runtime 1`] = `
+"import { _filter_ } from \\"@fink/js-interop/runtime.js\\";
+
+_filter_((item) => item % 2 === 0, 1, false, false);"
+`;
+
+exports[`filter compiles with default runtime 2`] = `
+"import { _filter_ } from \\"@fink/js-interop/runtime.js\\";
+
+_filter_((item, cntr = 0) => [item < 1 && cntr % 2 === 0, cntr + 1], 2, false, false);"
+`;
+
+exports[`filter legacy compiles 1`] = `
 "(function filter(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -64,7 +88,7 @@ exports[`filter compiles 1`] = `
 });"
 `;
 
-exports[`filter compiles with accu 1`] = `
+exports[`filter legacy compiles with accu 1`] = `
 "(function filter(ˆitems_5) {
   return {
     *[Symbol.iterator]() {
@@ -83,7 +107,7 @@ exports[`filter compiles with accu 1`] = `
 });"
 `;
 
-exports[`filter compiles with accu 2`] = `
+exports[`filter legacy compiles with accu 2`] = `
 "(function filter(ˆitems_5) {
   return {
     *[Symbol.iterator]() {
@@ -101,7 +125,7 @@ exports[`filter compiles with accu 2`] = `
 });"
 `;
 
-exports[`filter compiles with accu 3`] = `
+exports[`filter legacy compiles with accu 3`] = `
 "(function filter(ˆitems_5) {
   return {
     *[Symbol.iterator]() {
@@ -118,15 +142,3 @@ exports[`filter compiles with accu 3`] = `
   };
 });"
 `;
-
-exports[`gen 2 compiles 1`] = `
-"import { _filter_ } from \\"@fink/js-interop/iter.js\\";
-
-_filter_((item) => item % 2 === 0, 1, false, false);"
-`;
-
-exports[`gen 2 compiles 2`] = `
-"import { _filter_ } from \\"@fink/js-interop/iter.js\\";
-
-_filter_((item, cntr = 0) => [item < 1 && cntr % 2 === 0, cntr + 1], 2, false, false);"
-`;
diff --git a/src/lang/iterable/fold.test.fnk b/src/lang/iterable/fold.test.fnk
index f9be4bb..306f3d9 100644
--- a/src/lang/iterable/fold.test.fnk
+++ b/src/lang/iterable/fold.test.fnk
@@ -3,11 +3,10 @@
 
 
 
-describe 'gen 2', fn:
-  it 'compiles', fn:
+describe 'fold', fn:
+  it 'compiles with default runtime', fn:
     expect
       fink2js '
-        {_fold_} = import "@fink/js-interop/iter.js"
         fold item, prev=0:
           ni = item + prev
           item * ni
@@ -16,7 +15,6 @@ describe 'gen 2', fn:
 
     expect
       fink2js '
-        {_fold_} = import "@fink/js-interop/iter.js"
         fold item, prev=0, acc=1:
           [item * acc + prev, acc + 1]
       '
@@ -24,19 +22,42 @@ describe 'gen 2', fn:
 
     expect
       fink2js '
-        {_fold_} = import "@fink/js-interop/iter.js"
+        fold item, prev=0, acc=1, shared_acc:
+          [item * acc + prev, acc + 1, shared_acc + 1]
+      '
+      to_match_snapshot
+
+    expect
+      fink2js '
         fold item:
           item
       '
       to_match_snapshot
 
+    expect
+      fink2js '
+        fold: false
+      '
+      to_match_snapshot
+
 
+  it 'compiles with custom runtime', fn:
+    expect
+      fink2js '
+        {_fold_} = import "./fold.fnk"
+        fold item, prev=0:
+          ni = item + prev
+          item * ni
+      '
+      to_match_snapshot
 
 
-describe 'fold', fn:
+
+describe 'fold legacy', fn:
   it 'compiles', fn:
     expect
       fink2js '
+        {_fold_} = import "@fink/loxia:iter:legacy"
         fold item, prev=0:
           ni = item + prev
           item * ni
@@ -47,6 +68,7 @@ describe 'fold', fn:
    it 'compiles with accu', fn:
     expect
       fink2js '
+        {_fold_} = import "@fink/loxia:iter:legacy"
         fold item, prev=0, acc=1:
           [item * acc + prev, acc + 1]
       '
@@ -56,6 +78,7 @@ describe 'fold', fn:
   it 'compiles without using prev result', fn:
     expect
       fink2js '
+        {_fold_} = import "@fink/loxia:iter:legacy"
         fold item:
           item
       '
@@ -65,6 +88,7 @@ describe 'fold', fn:
   it 'destructuring item', fn:
     expect
       fink2js '
+        {_fold_} = import "@fink/loxia:iter:legacy"
         fold [foo, ...bar], prev=[]:
           [[foo, bar], ...prev]
 
@@ -77,6 +101,7 @@ describe 'fold', fn:
   it 'destructuring prev result', fn:
     expect
       fink2js '
+        {_fold_} = import "@fink/loxia:iter:legacy"
         fold item, [foo, ...bar]=[]:
           [item, bar, foo]
 
@@ -91,8 +116,10 @@ describe 'fold async', fn:
   it 'compiles', fn:
     expect
       fink2js '
+        {_fold_} = import "@fink/loxia:iter:legacy"
         fold item, acc=0:
           ni = await item + acc
           item * acc
       '
       to_match_snapshot
+
diff --git a/src/lang/iterable/fold.test.fnk.snap b/src/lang/iterable/fold.test.fnk.snap
index 70267d9..62907ee 100644
--- a/src/lang/iterable/fold.test.fnk.snap
+++ b/src/lang/iterable/fold.test.fnk.snap
@@ -16,7 +16,49 @@ exports[`fold async compiles 1`] = `
 };"
 `;
 
-exports[`fold compiles 1`] = `
+exports[`fold compiles with custom runtime 1`] = `
+"import { _fold_ } from \\"./fold.js\\";
+
+_fold_((item, prev = 0) => {
+  const ni = item + prev;
+  return item * ni;
+}, 2, false, false);"
+`;
+
+exports[`fold compiles with default runtime 1`] = `
+"import { _fold_ } from \\"@fink/js-interop/runtime.js\\";
+
+_fold_((item, prev = 0) => {
+  const ni = item + prev;
+  return item * ni;
+}, 2, false, false);"
+`;
+
+exports[`fold compiles with default runtime 2`] = `
+"import { _fold_ } from \\"@fink/js-interop/runtime.js\\";
+
+_fold_((item, prev = 0, acc = 1) => [item * acc + prev, acc + 1], 3, false, false);"
+`;
+
+exports[`fold compiles with default runtime 3`] = `
+"import { _fold_ } from \\"@fink/js-interop/runtime.js\\";
+
+_fold_((item, prev = 0, acc = 1, shared_acc) => [item * acc + prev, acc + 1, shared_acc + 1], 4, false, false);"
+`;
+
+exports[`fold compiles with default runtime 4`] = `
+"import { _fold_ } from \\"@fink/js-interop/runtime.js\\";
+
+_fold_((item) => item, 1, false, false);"
+`;
+
+exports[`fold compiles with default runtime 5`] = `
+"import { _fold_ } from \\"@fink/js-interop/runtime.js\\";
+
+_fold_(() => false, 0, false, false);"
+`;
+
+exports[`fold legacy compiles 1`] = `
 "ˆitems_3 => {
   let ˆfold_result_4 = 0;
 
@@ -32,7 +74,7 @@ exports[`fold compiles 1`] = `
 };"
 `;
 
-exports[`fold compiles with accu 1`] = `
+exports[`fold legacy compiles with accu 1`] = `
 "ˆitems_5 => {
   let ˆaccu_2 = 1;
   let ˆfold_result_6 = 0;
@@ -50,7 +92,7 @@ exports[`fold compiles with accu 1`] = `
 };"
 `;
 
-exports[`fold compiles without using prev result 1`] = `
+exports[`fold legacy compiles without using prev result 1`] = `
 "ˆitems_3 => {
   let ˆfold_result_4;
 
@@ -64,7 +106,7 @@ exports[`fold compiles without using prev result 1`] = `
 };"
 `;
 
-exports[`fold destructuring item 1`] = `
+exports[`fold legacy destructuring item 1`] = `
 "ˆitems_3 => {
   let ˆfold_result_4 = [];
 
@@ -99,7 +141,7 @@ exports[`fold destructuring item 1`] = `
 };"
 `;
 
-exports[`fold destructuring prev result 1`] = `
+exports[`fold legacy destructuring prev result 1`] = `
 "ˆitems_3 => {
   let ˆfold_result_4 = [];
 
@@ -134,24 +176,3 @@ exports[`fold destructuring prev result 1`] = `
   return ˆfold_result_8;
 };"
 `;
-
-exports[`gen 2 compiles 1`] = `
-"import { _fold_ } from \\"@fink/js-interop/iter.js\\";
-
-_fold_((item, prev = 0) => {
-  const ni = item + prev;
-  return item * ni;
-}, 2, false, false);"
-`;
-
-exports[`gen 2 compiles 2`] = `
-"import { _fold_ } from \\"@fink/js-interop/iter.js\\";
-
-_fold_((item, prev = 0, acc = 1) => [item * acc + prev, acc + 1], 3, false, false);"
-`;
-
-exports[`gen 2 compiles 3`] = `
-"import { _fold_ } from \\"@fink/js-interop/iter.js\\";
-
-_fold_((item) => item, 1, false, false);"
-`;
diff --git a/src/lang/iterable/init.fnk b/src/lang/iterable/init.fnk
index 9207bde..575ae9c 100644
--- a/src/lang/iterable/init.fnk
+++ b/src/lang/iterable/init.fnk
@@ -1,4 +1,4 @@
-{add_with_builtin, any} = import '../context.fnk'
+{add_with_runtime, any} = import '../context.fnk'
 
 {transform_fold} = import './fold.fnk'
 {transform_unfold} = import './unfold.fnk'
@@ -31,7 +31,7 @@ with_legacy = fn legacy_transform: fn node, ctx:
 
 add = fn op, legacy_transform: fn ctx:
   pipe ctx:
-    add_with_builtin any, op, legacy_uri, with_legacy legacy_transform
+    add_with_runtime any, op, with_legacy legacy_transform
 
 
 
diff --git a/src/lang/iterable/map.test.fnk b/src/lang/iterable/map.test.fnk
index 42b184f..85e33c0 100644
--- a/src/lang/iterable/map.test.fnk
+++ b/src/lang/iterable/map.test.fnk
@@ -3,53 +3,45 @@
 {fink2js} = import '../../testing/generate.fnk'
 
 
-describe 'gen 2', fn:
-  it 'compiles', fn:
-    expect
-      fink2js '
-        {_map_} = import "@fink/js-interop/iter.js"
-        map item:
-          item * 2
-      '
-      to_match_snapshot
 
+describe 'map', fn:
+
+  it 'uses default runtime', fn:
     expect
       fink2js '
-        {_map_} = import "@fink/js-interop/iter.js"
-        map item, acc=0:
-          [item * 2 * acc, acc + 1]
-      '
-      to_match_snapshot
+        pipe [1, 2, 3]:
+          map item:
+            item * 2
 
+          map item, acc=0:
+            [item + acc, acc + 1]
 
-  it 'uses custom iterable impl.', fn:
-    expect
-      fink2js '
-        {_map_} = import "@fink/js-interop/iter.js"
-        map item:
-          ...[item, item * 2]
+          map item, acc=0, shared_acc=0:
+            [item + acc + shared_acc, acc + 1, shared_acc + 1]
+
+          map item:
+            ...item
       '
       to_match_snapshot
 
 
-
-  it 'uses multiple custom iterable impl.', fn:
+  it 'handles await', fn:
     expect
       fink2js '
-        {_map_} = import "./iter.fnk"
-        {_filter_} = import "./iter-filter.fnk"
         pipe [1, 2, 3]:
-          map item: item * 2
-          map item: item / 2
-          filter item: item % 2 == 0
+          map await item:
+            item * 2
       '
       to_match_snapshot
 
 
-  it 'uses legacy iterables', fn:
+describe 'custom runtime', fn:
+
+  it 'uses multiple custom runtimes', fn:
     expect
       fink2js '
-        {_map_, _filter_} = import "@fink/loxia:iter:legacy"
+        {_map_} = import "./iter.fnk"
+        {_filter_} = import "./iter-filter.fnk"
         pipe [1, 2, 3]:
           map item: item * 2
           map item: item / 2
@@ -59,10 +51,11 @@ describe 'gen 2', fn:
 
 
 
-describe 'map', fn:
+describe 'map legacy', fn:
   it 'compiles single line', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map item: item * 2
       '
       to_match_snapshot
@@ -71,6 +64,7 @@ describe 'map', fn:
   it 'compiles multi line', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map item:
           ni = foo item
           ni + 2
@@ -81,6 +75,7 @@ describe 'map', fn:
   it 'compiles single line with default value', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map item=123: item * 2
       '
       to_match_snapshot
@@ -89,6 +84,7 @@ describe 'map', fn:
   it 'compiles single line with destructured obj', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map {item}: item * 2
       '
       to_match_snapshot
@@ -97,6 +93,7 @@ describe 'map', fn:
   it 'compiles single line with destructured list', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map [x, y]: x + y
       '
       to_match_snapshot
@@ -105,6 +102,7 @@ describe 'map', fn:
   it 'compiles as flat map', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map [x, y]:
           ...[x, y]
       '
@@ -114,6 +112,7 @@ describe 'map', fn:
   it 'compiles with foo', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map {foo}:
           match foo:
             bar: spam
@@ -124,6 +123,7 @@ describe 'map', fn:
   it 'compiles with acc', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map {foo}, acc=0:
           [foo + acc, acc+1]
       '
@@ -131,6 +131,7 @@ describe 'map', fn:
 
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map {foo}, acc=0:
           ...[[acc, foo], acc+1]
       '
@@ -140,6 +141,7 @@ describe 'map', fn:
   it 'compiles destructuring', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map [x, ...rest]:
          rest
 
@@ -149,12 +151,13 @@ describe 'map', fn:
       to_match_snapshot
 
 
-
-describe 'map async', fn:
-  it 'compiles', fn:
+  it 'compiles async', fn:
     expect
       fink2js '
+        {_map_} = import "@fink/loxia:iter:legacy"
         map item:
           await item * 2
       '
-      to_match_snapshot
\ No newline at end of file
+      to_match_snapshot
+
+
diff --git a/src/lang/iterable/map.test.fnk.snap b/src/lang/iterable/map.test.fnk.snap
index d2e0560..e118942 100644
--- a/src/lang/iterable/map.test.fnk.snap
+++ b/src/lang/iterable/map.test.fnk.snap
@@ -1,69 +1,6 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`gen 2 compiles 1`] = `
-"import { _map_ } from \\"@fink/js-interop/iter.js\\";
-
-_map_((item) => item * 2, 1, false, false);"
-`;
-
-exports[`gen 2 compiles 2`] = `
-"import { _map_ } from \\"@fink/js-interop/iter.js\\";
-
-_map_((item, acc = 0) => [item * 2 * acc, acc + 1], 2, false, false);"
-`;
-
-exports[`gen 2 uses custom iterable impl. 1`] = `
-"import { _map_ } from \\"@fink/js-interop/iter.js\\";
-
-_map_((item) => [item, item * 2], 1, false, true);"
-`;
-
-exports[`gen 2 uses legacy iterables 1`] = `
-"{
-  let ˆpipe_result_1 = [1, 2, 3];
-
-  ˆpipe_result_1 = function map(ˆitems_4) {
-    return {
-      *[Symbol.iterator]() {
-        for (const ˆitem_2 of ˆitems_4) {
-          const item = ˆitem_2;
-          const ˆresult_3 = item * 2;
-          yield ˆresult_3;
-        }
-      }
-
-    };
-  }(ˆpipe_result_1);
-
-  ˆpipe_result_1 = function map(ˆitems_7) {
-    return {
-      *[Symbol.iterator]() {
-        for (const ˆitem_5 of ˆitems_7) {
-          const item = ˆitem_5;
-          const ˆresult_6 = item / 2;
-          yield ˆresult_6;
-        }
-      }
-
-    };
-  }(ˆpipe_result_1);
-
-  ˆpipe_result_1 = function filter(ˆitems_10) {
-    return {
-      *[Symbol.iterator]() {
-        for (const ˆitem_8 of ˆitems_10) {
-          const item = ˆitem_8;
-          const ˆresult_9 = item % 2 === 0;
-          if (ˆresult_9) yield ˆitem_8;
-        }
-      }
-
-    };
-  }(ˆpipe_result_1);
-}"
-`;
-
-exports[`gen 2 uses multiple custom iterable impl. 1`] = `
+exports[`custom runtime uses multiple custom runtimes 1`] = `
 "import { _map_ } from \\"./iter.js\\";
 import { _filter_ } from \\"./iter-filter.js\\";
 {
@@ -74,14 +11,22 @@ import { _filter_ } from \\"./iter-filter.js\\";
 }"
 `;
 
-exports[`map async compiles 1`] = `
+exports[`map handles await 1`] = `
+"import { _map_ } from \\"@fink/js-interop/runtime.js\\";
+{
+  let ˆpipe_result_1 = [1, 2, 3];
+  ˆpipe_result_1 = _map_((item) => item * 2, 1, true, false)(ˆpipe_result_1);
+}"
+`;
+
+exports[`map legacy compiles as flat map 1`] = `
 "(function map(ˆitems_3) {
   return {
-    async *[Symbol.asyncIterator]() {
-      for await (const ˆitem_1 of ˆitems_3) {
-        const item = ˆitem_1;
-        const ˆresult_2 = (await item) * 2;
-        yield ˆresult_2;
+    *[Symbol.iterator]() {
+      for (const ˆitem_1 of ˆitems_3) {
+        const [x, y] = ˆitem_1;
+        const ˆresult_2 = [x, y];
+        yield* ˆresult_2;
       }
     }
 
@@ -89,14 +34,14 @@ exports[`map async compiles 1`] = `
 });"
 `;
 
-exports[`map compiles as flat map 1`] = `
+exports[`map legacy compiles async 1`] = `
 "(function map(ˆitems_3) {
   return {
-    *[Symbol.iterator]() {
-      for (const ˆitem_1 of ˆitems_3) {
-        const [x, y] = ˆitem_1;
-        const ˆresult_2 = [x, y];
-        yield* ˆresult_2;
+    async *[Symbol.asyncIterator]() {
+      for await (const ˆitem_1 of ˆitems_3) {
+        const item = ˆitem_1;
+        const ˆresult_2 = (await item) * 2;
+        yield ˆresult_2;
       }
     }
 
@@ -104,7 +49,7 @@ exports[`map compiles as flat map 1`] = `
 });"
 `;
 
-exports[`map compiles destructuring 1`] = `
+exports[`map legacy compiles destructuring 1`] = `
 "(function map(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -139,7 +84,7 @@ exports[`map compiles destructuring 1`] = `
 });"
 `;
 
-exports[`map compiles multi line 1`] = `
+exports[`map legacy compiles multi line 1`] = `
 "(function map(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -155,7 +100,7 @@ exports[`map compiles multi line 1`] = `
 });"
 `;
 
-exports[`map compiles single line 1`] = `
+exports[`map legacy compiles single line 1`] = `
 "(function map(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -170,7 +115,7 @@ exports[`map compiles single line 1`] = `
 });"
 `;
 
-exports[`map compiles single line with default value 1`] = `
+exports[`map legacy compiles single line with default value 1`] = `
 "(function map(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -185,7 +130,7 @@ exports[`map compiles single line with default value 1`] = `
 });"
 `;
 
-exports[`map compiles single line with destructured list 1`] = `
+exports[`map legacy compiles single line with destructured list 1`] = `
 "(function map(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -200,7 +145,7 @@ exports[`map compiles single line with destructured list 1`] = `
 });"
 `;
 
-exports[`map compiles single line with destructured obj 1`] = `
+exports[`map legacy compiles single line with destructured obj 1`] = `
 "(function map(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -217,7 +162,7 @@ exports[`map compiles single line with destructured obj 1`] = `
 });"
 `;
 
-exports[`map compiles with acc 1`] = `
+exports[`map legacy compiles with acc 1`] = `
 "(function map(ˆitems_5) {
   return {
     *[Symbol.iterator]() {
@@ -238,7 +183,7 @@ exports[`map compiles with acc 1`] = `
 });"
 `;
 
-exports[`map compiles with acc 2`] = `
+exports[`map legacy compiles with acc 2`] = `
 "(function map(ˆitems_5) {
   return {
     *[Symbol.iterator]() {
@@ -259,7 +204,7 @@ exports[`map compiles with acc 2`] = `
 });"
 `;
 
-exports[`map compiles with foo 1`] = `
+exports[`map legacy compiles with foo 1`] = `
 "(function map(ˆitems_5) {
   return {
     *[Symbol.iterator]() {
@@ -288,3 +233,14 @@ exports[`map compiles with foo 1`] = `
   };
 });"
 `;
+
+exports[`map uses default runtime 1`] = `
+"import { _map_ } from \\"@fink/js-interop/runtime.js\\";
+{
+  let ˆpipe_result_1 = [1, 2, 3];
+  ˆpipe_result_1 = _map_((item) => item * 2, 1, false, false)(ˆpipe_result_1);
+  ˆpipe_result_1 = _map_((item, acc = 0) => [item + acc, acc + 1], 2, false, false)(ˆpipe_result_1);
+  ˆpipe_result_1 = _map_((item, acc = 0, shared_acc = 0) => [item + acc + shared_acc, acc + 1, shared_acc + 1], 3, false, false)(ˆpipe_result_1);
+  ˆpipe_result_1 = _map_((item) => item, 1, false, true)(ˆpipe_result_1);
+}"
+`;
diff --git a/src/lang/iterable/unfold.test.fnk b/src/lang/iterable/unfold.test.fnk
index 95a8571..f33d34d 100644
--- a/src/lang/iterable/unfold.test.fnk
+++ b/src/lang/iterable/unfold.test.fnk
@@ -3,11 +3,17 @@
 
 
 
-describe 'gen 2', fn:
-  it 'compiles', fn:
+describe 'unfold', fn:
+  it 'compiles with default runtime', fn:
+    expect
+      fink2js '
+        unfold :
+          123
+      '
+      to_match_snapshot
+
     expect
       fink2js '
-        {_unfold_} = import "@fink/js-interop/iter.js"
         unfold prev:
           prev + 1
       '
@@ -15,7 +21,6 @@ describe 'gen 2', fn:
 
     expect
       fink2js '
-        {_unfold_} = import "@fink/js-interop/iter.js"
         unfold prev=1:
           prev + 1
       '
@@ -23,21 +28,37 @@ describe 'gen 2', fn:
 
     expect
       fink2js '
-        {_unfold_} = import "@fink/js-interop/iter.js"
         unfold prev=1, acc=1:
           [prev * accu, accu + 1]
       '
       to_match_snapshot
 
+    expect
+      fink2js '
+        unfold prev=1, acc=1, shared_accu:
+          [prev * accu, accu + 1, shared_accu + 1]
+      '
+      to_match_snapshot
 
 
+  it 'compiles with custom runtime', fn:
+    expect
+      fink2js '
+        {_unfold_} = import "./iter.fnk"
+        unfold prev:
+          prev + 1
+      '
+      to_match_snapshot
 
 
-describe 'unfold', fn:
+
+
+describe 'unfold legacy', fn:
 
   it 'compiles with previous', fn:
     expect
       fink2js '
+        {_unfold_} = import "@fink/loxia:iter:legacy"
         unfold prev:
           prev + 1
       '
@@ -47,6 +68,7 @@ describe 'unfold', fn:
   it 'compiles with default', fn:
     expect
       fink2js '
+        {_unfold_} = import "@fink/loxia:iter:legacy"
         unfold prev=1:
           prev + 1
       '
@@ -56,6 +78,7 @@ describe 'unfold', fn:
   it 'compiles with default and accu', fn:
     expect
       fink2js '
+        {_unfold_} = import "@fink/loxia:iter:legacy"
         unfold prev=1, acc=1:
           [prev * accu, accu + 1]
       '
@@ -65,6 +88,7 @@ describe 'unfold', fn:
   it 'compiles without args', fn:
     expect
       fink2js '
+        {_unfold_} = import "@fink/loxia:iter:legacy"
         unfold:
           1234
       '
@@ -74,6 +98,7 @@ describe 'unfold', fn:
   it 'compiles with spread', fn:
     expect
       fink2js '
+        {_unfold_} = import "@fink/loxia:iter:legacy"
         unfold:
           ...[1, 2, 3]
       '
diff --git a/src/lang/iterable/unfold.test.fnk.snap b/src/lang/iterable/unfold.test.fnk.snap
index 2a00b7f..e56dee1 100644
--- a/src/lang/iterable/unfold.test.fnk.snap
+++ b/src/lang/iterable/unfold.test.fnk.snap
@@ -1,24 +1,42 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`gen 2 compiles 1`] = `
-"import { _unfold_ } from \\"@fink/js-interop/iter.js\\";
+exports[`unfold compiles with custom runtime 1`] = `
+"import { _unfold_ } from \\"./iter.js\\";
 
 _unfold_((prev) => prev + 1, 1, false, false);"
 `;
 
-exports[`gen 2 compiles 2`] = `
-"import { _unfold_ } from \\"@fink/js-interop/iter.js\\";
+exports[`unfold compiles with default runtime 1`] = `
+"import { _unfold_ } from \\"@fink/js-interop/runtime.js\\";
+
+_unfold_(() => 123, 0, false, false);"
+`;
+
+exports[`unfold compiles with default runtime 2`] = `
+"import { _unfold_ } from \\"@fink/js-interop/runtime.js\\";
+
+_unfold_((prev) => prev + 1, 1, false, false);"
+`;
+
+exports[`unfold compiles with default runtime 3`] = `
+"import { _unfold_ } from \\"@fink/js-interop/runtime.js\\";
 
 _unfold_((prev = 1) => prev + 1, 1, false, false);"
 `;
 
-exports[`gen 2 compiles 3`] = `
-"import { _unfold_ } from \\"@fink/js-interop/iter.js\\";
+exports[`unfold compiles with default runtime 4`] = `
+"import { _unfold_ } from \\"@fink/js-interop/runtime.js\\";
 
 _unfold_((prev = 1, acc = 1) => [prev * accu, accu + 1], 2, false, false);"
 `;
 
-exports[`unfold compiles with default 1`] = `
+exports[`unfold compiles with default runtime 5`] = `
+"import { _unfold_ } from \\"@fink/js-interop/runtime.js\\";
+
+_unfold_((prev = 1, acc = 1, shared_accu) => [prev * accu, accu + 1, shared_accu + 1], 3, false, false);"
+`;
+
+exports[`unfold legacy compiles with default 1`] = `
 "(function unfold(ˆinitial_3) {
   return {
     *[Symbol.iterator]() {
@@ -36,7 +54,7 @@ exports[`unfold compiles with default 1`] = `
 });"
 `;
 
-exports[`unfold compiles with default and accu 1`] = `
+exports[`unfold legacy compiles with default and accu 1`] = `
 "(function unfold(ˆinitial_5) {
   return {
     *[Symbol.iterator]() {
@@ -57,7 +75,7 @@ exports[`unfold compiles with default and accu 1`] = `
 });"
 `;
 
-exports[`unfold compiles with previous 1`] = `
+exports[`unfold legacy compiles with previous 1`] = `
 "(function unfold(ˆinitial_3) {
   return {
     *[Symbol.iterator]() {
@@ -75,7 +93,7 @@ exports[`unfold compiles with previous 1`] = `
 });"
 `;
 
-exports[`unfold compiles with spread 1`] = `
+exports[`unfold legacy compiles with spread 1`] = `
 "(function unfold(ˆinitial_3) {
   return {
     *[Symbol.iterator]() {
@@ -92,7 +110,7 @@ exports[`unfold compiles with spread 1`] = `
 });"
 `;
 
-exports[`unfold compiles without args 1`] = `
+exports[`unfold legacy compiles without args 1`] = `
 "(function unfold(ˆinitial_3) {
   return {
     *[Symbol.iterator]() {
diff --git a/src/lang/iterable/until.test.fnk b/src/lang/iterable/until.test.fnk
index 6416139..6f324d6 100644
--- a/src/lang/iterable/until.test.fnk
+++ b/src/lang/iterable/until.test.fnk
@@ -3,11 +3,10 @@
 
 
 
-describe 'gen 2', fn:
-  it 'compiles', fn:
+describe 'until', fn:
+  it 'compiles with default runtime', fn:
     expect
       fink2js '
-        {_until_} = import "@fink/js-interop/iter.js"
         until item:
           item < 10
       '
@@ -15,18 +14,34 @@ describe 'gen 2', fn:
 
     expect
       fink2js '
-        {_until_} = import "@fink/js-interop/iter.js"
         until item, cntr=0:
           [cntr > 2, cntr + 1]
       '
       to_match_snapshot
 
+    expect
+      fink2js '
+        until item, cntr=0, shared_accu:
+          [cntr > 2, cntr + 1, shared_accu + 1]
+      '
+      to_match_snapshot
 
 
-describe 'until', fn:
+  it 'compiles with custom runtime', fn:
+    fink2js '
+        {_until_} = import "./iter.fnk"
+        until item:
+          item < 10
+      '
+      to_match_snapshot
+
+
+
+describe 'until legacy', fn:
   it 'compiles', fn:
     expect
       fink2js '
+        {_until_} = import "@fink/loxia:iter:legacy"
         until item:
           item < 10
       '
@@ -36,6 +51,7 @@ describe 'until', fn:
   it 'compiles with accu', fn:
     expect
       fink2js '
+        {_until_} = import "@fink/loxia:iter:legacy"
         until _, cntr=0:
           [cntr > 2, cntr + 1]
 
@@ -48,6 +64,7 @@ describe 'until', fn:
   it 'compiles destructuring', fn:
     expect
       fink2js '
+        {_until_} = import "@fink/loxia:iter:legacy"
         until [foo, ...bar]:
           foo_alone = foo and is_empty bar
           foo_alone
@@ -63,6 +80,7 @@ describe 'until async', fn:
   it 'compiles', fn:
     expect
       fink2js '
+        {_until_} = import "@fink/loxia:iter:legacy"
         until item:
           await item < 10
       '
diff --git a/src/lang/iterable/until.test.fnk.snap b/src/lang/iterable/until.test.fnk.snap
index 5e220b9..506b7f5 100644
--- a/src/lang/iterable/until.test.fnk.snap
+++ b/src/lang/iterable/until.test.fnk.snap
@@ -1,17 +1,5 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`gen 2 compiles 1`] = `
-"import { _until_ } from \\"@fink/js-interop/iter.js\\";
-
-_until_((item) => item < 10, 1, false, false);"
-`;
-
-exports[`gen 2 compiles 2`] = `
-"import { _until_ } from \\"@fink/js-interop/iter.js\\";
-
-_until_((item, cntr = 0) => [cntr > 2, cntr + 1], 2, false, false);"
-`;
-
 exports[`until async compiles 1`] = `
 "(function filter_until(ˆitems_3) {
   return {
@@ -28,7 +16,25 @@ exports[`until async compiles 1`] = `
 });"
 `;
 
-exports[`until compiles 1`] = `
+exports[`until compiles with default runtime 1`] = `
+"import { _until_ } from \\"@fink/js-interop/runtime.js\\";
+
+_until_((item) => item < 10, 1, false, false);"
+`;
+
+exports[`until compiles with default runtime 2`] = `
+"import { _until_ } from \\"@fink/js-interop/runtime.js\\";
+
+_until_((item, cntr = 0) => [cntr > 2, cntr + 1], 2, false, false);"
+`;
+
+exports[`until compiles with default runtime 3`] = `
+"import { _until_ } from \\"@fink/js-interop/runtime.js\\";
+
+_until_((item, cntr = 0, shared_accu) => [cntr > 2, cntr + 1, shared_accu + 1], 3, false, false);"
+`;
+
+exports[`until legacy compiles 1`] = `
 "(function filter_until(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -44,7 +50,7 @@ exports[`until compiles 1`] = `
 });"
 `;
 
-exports[`until compiles destructuring 1`] = `
+exports[`until legacy compiles destructuring 1`] = `
 "(function filter_until(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -83,7 +89,7 @@ exports[`until compiles destructuring 1`] = `
 });"
 `;
 
-exports[`until compiles with accu 1`] = `
+exports[`until legacy compiles with accu 1`] = `
 "(function filter_until(ˆitems_5) {
   return {
     *[Symbol.iterator]() {
diff --git a/src/lang/iterable/while.test.fnk b/src/lang/iterable/while.test.fnk
index 94559d9..6eeafa7 100644
--- a/src/lang/iterable/while.test.fnk
+++ b/src/lang/iterable/while.test.fnk
@@ -3,11 +3,10 @@
 
 
 
-describe 'gen 2', fn:
-  it 'compiles', fn:
+describe 'while', fn:
+  it 'compiles with default runtime', fn:
     expect
       fink2js '
-        {_while_} = import "@fink/js-interop/iter.js"
         while item:
           item < 10
       '
@@ -15,19 +14,34 @@ describe 'gen 2', fn:
 
     expect
       fink2js '
-        {_while_} = import "@fink/js-interop/iter.js"
         while item, cntr=0:
           [cntr > 2, cntr + 1]
       '
       to_match_snapshot
 
+    expect
+      fink2js '
+        while item, cntr=0, shared_accu:
+          [cntr > 2, cntr + 1, shared_accu + 1]
+      '
+      to_match_snapshot
 
 
+  it 'compiles with custom runtime', fn:
+    fink2js '
+      {_while_} = import "./iter.fnk"
+      while item:
+          item < 10
+    '
+    to_match_snapshot
 
-describe 'while', fn:
+
+
+describe 'while legacy', fn:
   it 'compiles', fn:
     expect
       fink2js '
+        {_while_} = import "@fink/loxia:iter:legacy"
         while item:
           item < 10
       '
@@ -37,6 +51,8 @@ describe 'while', fn:
   it 'compiles with accu', fn:
     expect
       fink2js '
+        {_while_} = import "@fink/loxia:iter:legacy"
+
         while _, cntr=0:
           [cntr < 2, cntr + 1]
 
@@ -52,6 +68,7 @@ describe 'while', fn:
   it 'compiles destructuring', fn:
     expect
       fink2js '
+        {_while_} = import "@fink/loxia:iter:legacy"
         while [foo, ...bar]:
           foo_alone = foo and is_empty bar
           foo_alone
@@ -67,6 +84,7 @@ describe 'while async', fn:
   it 'compiles', fn:
     expect
       fink2js '
+        {_while_} = import "@fink/loxia:iter:legacy"
         while item:
           await item < 10
       '
diff --git a/src/lang/iterable/while.test.fnk.snap b/src/lang/iterable/while.test.fnk.snap
index e43dcea..a75c6f8 100644
--- a/src/lang/iterable/while.test.fnk.snap
+++ b/src/lang/iterable/while.test.fnk.snap
@@ -1,17 +1,5 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`gen 2 compiles 1`] = `
-"import { _while_ } from \\"@fink/js-interop/iter.js\\";
-
-_while_((item) => item < 10, 1, false, false);"
-`;
-
-exports[`gen 2 compiles 2`] = `
-"import { _while_ } from \\"@fink/js-interop/iter.js\\";
-
-_while_((item, cntr = 0) => [cntr > 2, cntr + 1], 2, false, false);"
-`;
-
 exports[`while async compiles 1`] = `
 "(function filter_while(ˆitems_3) {
   return {
@@ -28,7 +16,25 @@ exports[`while async compiles 1`] = `
 });"
 `;
 
-exports[`while compiles 1`] = `
+exports[`while compiles with default runtime 1`] = `
+"import { _while_ } from \\"@fink/js-interop/runtime.js\\";
+
+_while_((item) => item < 10, 1, false, false);"
+`;
+
+exports[`while compiles with default runtime 2`] = `
+"import { _while_ } from \\"@fink/js-interop/runtime.js\\";
+
+_while_((item, cntr = 0) => [cntr > 2, cntr + 1], 2, false, false);"
+`;
+
+exports[`while compiles with default runtime 3`] = `
+"import { _while_ } from \\"@fink/js-interop/runtime.js\\";
+
+_while_((item, cntr = 0, shared_accu) => [cntr > 2, cntr + 1, shared_accu + 1], 3, false, false);"
+`;
+
+exports[`while legacy compiles 1`] = `
 "(function filter_while(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -44,7 +50,7 @@ exports[`while compiles 1`] = `
 });"
 `;
 
-exports[`while compiles destructuring 1`] = `
+exports[`while legacy compiles destructuring 1`] = `
 "(function filter_while(ˆitems_3) {
   return {
     *[Symbol.iterator]() {
@@ -82,7 +88,7 @@ exports[`while compiles destructuring 1`] = `
 });"
 `;
 
-exports[`while compiles with accu 1`] = `
+exports[`while legacy compiles with accu 1`] = `
 "(function filter_while(ˆitems_5) {
   return {
     *[Symbol.iterator]() {
diff --git a/src/lang/logical/in.fnk b/src/lang/logical/in.fnk
index cda5f43..9021b54 100644
--- a/src/lang/logical/in.fnk
+++ b/src/lang/logical/in.fnk
@@ -10,8 +10,6 @@ babel_types = import '@babel/types'
 
 legacy_uri = '@fink/loxia:iter:legacy'
 
-default_in_impl_uri = legacy_uri
-
 
 
 transform_legacy = fn node, ctx:
@@ -82,6 +80,7 @@ transform_legacy = fn node, ctx:
 
 
 transform_with_runtime_lib = fn runtime_fn, node, ctx:
+  # TODO: if right hand side is a literal, we could optimize as legacy did
   [right, next_ctx] = transform node.right, ctx
   [left, end_ctx] = transform node.left, next_ctx
 
@@ -100,7 +99,6 @@ transform_in = fn node, ctx:
   match next_ctx:
     legacy_uri == runtime_impl runtime_fn, ?:
       transform_legacy node, next_ctx
-
     else:
       transform_with_runtime_lib runtime_fn, node, next_ctx
 
diff --git a/src/lang/logical/in.test.fnk b/src/lang/logical/in.test.fnk
index abbca1c..f06cbe7 100644
--- a/src/lang/logical/in.test.fnk
+++ b/src/lang/logical/in.test.fnk
@@ -3,7 +3,7 @@
 
 
 describe 'in', fn:
-  it 'shortcuts known with knwon right hand side', fn:
+  it 'compiles with default runtime', fn:
     expect
       fink2js "
         foo = 1 in [1, 2, 3]
@@ -14,10 +14,10 @@ describe 'in', fn:
       to_match_snapshot
 
 
-  it 'compiles with explicit legacy impl', fn:
+  it 'compiles with custom runtime', fn:
     expect
       fink2js "
-        {_in_} = import '@fink/loxia:iter:legacy'
+        {_in_} = import './foo.fnk'
         foo = 1 in [1, 2, 3]
         bar = '2' in '1234'
         spam = key in {foo: bar}
@@ -26,13 +26,16 @@ describe 'in', fn:
       to_match_snapshot
 
 
-  it 'compiles with custom builin lib', fn:
+  it 'compiles with legacy', fn:
     expect
       fink2js "
-        {_in_} = import './foo.fnk'
+        {_in_} = import '@fink/loxia:iter:legacy'
         foo = 1 in [1, 2, 3]
         bar = '2' in '1234'
         spam = key in {foo: bar}
         shrub = item in ni
       "
       to_match_snapshot
+
+
+
diff --git a/src/lang/logical/in.test.fnk.snap b/src/lang/logical/in.test.fnk.snap
index 48b5591..b1a3854 100644
--- a/src/lang/logical/in.test.fnk.snap
+++ b/src/lang/logical/in.test.fnk.snap
@@ -1,6 +1,6 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
-exports[`in compiles with custom builin lib 1`] = `
+exports[`in compiles with custom runtime 1`] = `
 "import { _in_ } from \\"./foo.js\\";
 export const foo = _in_(1, [1, 2, 3]);
 export const bar = _in_(\`2\`, \`1234\`);
@@ -10,16 +10,17 @@ export const spam = _in_(key, {
 export const shrub = _in_(item, ni);"
 `;
 
-exports[`in compiles with explicit legacy impl 1`] = `
-"export const foo = [1, 2, 3].includes(1);
-export const bar = \`1234\`.includes(\`2\`);
-export const spam = {
+exports[`in compiles with default runtime 1`] = `
+"import { _in_ } from \\"@fink/js-interop/runtime.js\\";
+export const foo = _in_(1, [1, 2, 3]);
+export const bar = _in_(\`2\`, \`1234\`);
+export const spam = _in_(key, {
   foo: bar
-}.hasOwnProperty(key);
-export const shrub = ni?.includes?.(item) ?? ni?.has?.(item) ?? ni?.hasOwnProperty?.(item) ?? false;"
+});
+export const shrub = _in_(item, ni);"
 `;
 
-exports[`in shortcuts known with knwon right hand side 1`] = `
+exports[`in compiles with legacy 1`] = `
 "export const foo = [1, 2, 3].includes(1);
 export const bar = \`1234\`.includes(\`2\`);
 export const spam = {
diff --git a/src/lang/logical/init.fnk b/src/lang/logical/init.fnk
index 3ea5636..db7979e 100644
--- a/src/lang/logical/init.fnk
+++ b/src/lang/logical/init.fnk
@@ -1,10 +1,10 @@
 babel_types = import '@babel/types'
 {logicalExpression, unaryExpression} = babel_types
 
-{add, add_with_builtin, any} = import '../context.fnk'
+{add, add_with_runtime, any} = import '../context.fnk'
 {transform} = import '../transform.fnk'
 
-{transform_in, default_in_impl_uri} = import './in.fnk'
+{transform_in} = import './in.fnk'
 
 
 
@@ -40,4 +40,4 @@ add_logical = fn ctx:
     add any, 'or', transform_logical
     add any, 'not', transform_not
     # TODO: should this live in iterables?
-    add_with_builtin any, 'in', default_in_impl_uri, transform_in
+    add_with_runtime any, 'in', transform_in
diff --git a/src/lang/runtime.fnk b/src/lang/runtime.fnk
index 0406f80..465990f 100644
--- a/src/lang/runtime.fnk
+++ b/src/lang/runtime.fnk
@@ -1,6 +1,8 @@
 {is_str} = import '@fink/std-lib/str.fnk'
 
 
+default_in_runtime = '@fink/js-interop/runtime.js'
+
 
 add_runtime_fn = fn name, ctx:
   {runtime} = ctx
@@ -12,7 +14,7 @@ add_runtime_fn = fn name, ctx:
 
 
 
-set_runtime_impl = fn name, uri, ctx:
+set_runtime_impl = fn name, uri=default_in_runtime, ctx:
   {runtime} = ctx
 
   overrides = match runtime:
@@ -82,7 +84,7 @@ get_runtime_imports = fn ctx:
 
 
 
-init_builtins = fn ctx:
+init_runtimes = fn ctx:
   rec:
     ...ctx
     runtime: rec: