doc: editing models

doc/editing_models.md

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+# Editing Models
+
+## On
+
+In the "On" model, the cursor is _on_ a node. Insertion is complicated, because
+the cursor can't go all of the places you might want to insert a node, such as
+after all nodes, or inside an empty list. For this reason, "adding stuff" is
+split into two phases:
+
+1. i/a/o -- insert before,after,child adds a hole
+2. r/p -- replace the hole, or paste over the hole
+
+**Advantages:** easy to display.
+
+**Disadvantages:** _requires_ i/a/o for editing.
+
+## Phantom
+
+A variant of "on" where there's a "phantom node" at the end of every listy
+sibling list. That way you can navigate to it to insert.
+
+**Advantages:** easy to display.
+
+**Disadvantages:** display is hard: either you always show it (a completely
+unacceptable amount of noise) or you show it only when the list is selected
+(producing "flickering").
+
+## Between
+
+The cursor is _between_ nodes. Insertion and deletion works like inserting and
+deleting single characters in a regular text editor. Thus you only need one
+basic insertion operation, and you don't need holes in listy nodes, and in fact
+_never need to insert a hole_ because they only need show up in incomplete fixed
+nodes.
+
+What does down do? If it goes to the beginning of the node to the left, then
+"down down" never works. On the other hand, if you just inserted a node, its now
+to your left, and you may want to go to its beginning. One nice thing for "down"
+to do is to go to the end of the previous node or the beginning of the next
+node, both of which are adjacent to the cursor.
+
+Display is hard. Some options:
+
+- Highlight the previous node, drawing a bar to its right, unless you're at the
+  beginning in which case you highlight the next node and draw a bar to its
+  right, unless you're at an empty in which case you do something else. The
+  downsides of this are its complexity, and the fact that it's a lie: your
+  cursor isn't _on_ a node, it's _between_ nodes.
+- Highlight the previous node in gray and the next node in black. Works nicely
+  for everything but empty lists, where you perhaps insert a single black
+  square, maybe with a cdot.
+
+**Advantages:** fewer required operations, no holes needed.
+**Disadvantages:** hard to display, what does down do.
+
+## Comparison of edits
+
+    ~On~
+    i/I - insert hole before/beginning
+    a/A - insert hole after/end
+    o/O - insert hole as first/last child
+    r - replace node with construct
+    d - remove node
+    x - cut node
+    p - paste over node
+    j/J - first/last child
+
+    ~Between~
+    ^/$ - goto beginning/end of siblings
+    o/O - goto beginning/end of children
+    k - goto beginning of left node
+    r - insert construct
+    d - remove/delete
+    x - cut
+    p - paste before
+    j/J - end of left node / beginning of right node
+
+     On        | Between   | Operation
+    -----------+-----------+---------------------
+     Ap        | $p        | paste at end of list
+     arlorl    | rljrl     | [[]]
+     ar+jr1lr2 | r+kr1r2   | 1 + 2
+     xap       | xlp       | swap