diff --git a/ogma-core/CHANGELOG.md b/ogma-core/CHANGELOG.md
index 2f2f161..c2bafd4 100644
--- a/ogma-core/CHANGELOG.md
+++ b/ogma-core/CHANGELOG.md
@@ -1,8 +1,9 @@
 # Revision history for ogma-core
 
-## [1.X.Y] - 2023-05-20
+## [1.X.Y] - 2023-05-21
 
 * Allow customizing the names of the C files generated by Copilot (#80).
+* Translate ZtoPre and YtoPre to Copilot (#86).
 
 ## [1.0.8] - 2023-03-21
 
diff --git a/ogma-core/src/Language/Trans/CoCoSpec2Copilot.hs b/ogma-core/src/Language/Trans/CoCoSpec2Copilot.hs
index f0894b9..6a73cfa 100644
--- a/ogma-core/src/Language/Trans/CoCoSpec2Copilot.hs
+++ b/ogma-core/src/Language/Trans/CoCoSpec2Copilot.hs
@@ -130,6 +130,8 @@ const2Copilot BoolConstFTP   = "ftp"
 -- operator.
 opOnePre2Copilot :: Op1Pre -> String
 opOnePre2Copilot Op1Pre    = "pre"
+opOnePre2Copilot Op1YtoPre = "pre"
+opOnePre2Copilot Op1ZtoPre = "tpre"
 opOnePre2Copilot Op1Once   = "PTLTL.eventuallyPrev"
 opOnePre2Copilot Op1Hist   = "PTLTL.alwaysBeen"
 opOnePre2Copilot Op1Y      = "PTLTL.previous"
diff --git a/ogma-core/src/Language/Trans/FRETComponentSpec2Copilot.hs b/ogma-core/src/Language/Trans/FRETComponentSpec2Copilot.hs
index 51ae667..d262997 100644
--- a/ogma-core/src/Language/Trans/FRETComponentSpec2Copilot.hs
+++ b/ogma-core/src/Language/Trans/FRETComponentSpec2Copilot.hs
@@ -87,6 +87,7 @@ fretComponentSpec2Copilot' prefs fretComponentSpec =
       , pure clock
       , pure ftp
       , pure pre
+      , pure tpre
       , pure spec
       , pure main'
       ]
@@ -223,6 +224,12 @@ fretComponentSpec2Copilot' prefs fretComponentSpec =
           , "pre = ([False] ++)"
           ]
 
+    -- Auxiliary streams: tpre
+    tpre = [ ""
+           , "tpre :: Stream Bool -> Stream Bool"
+           , "tpre = ([True] ++)"
+           ]
+
     -- Main specification
     spec :: [String]
     spec = [ ""
diff --git a/ogma-core/src/Language/Trans/FRETReqsDB2Copilot.hs b/ogma-core/src/Language/Trans/FRETReqsDB2Copilot.hs
index ef8a61e..72e447c 100644
--- a/ogma-core/src/Language/Trans/FRETReqsDB2Copilot.hs
+++ b/ogma-core/src/Language/Trans/FRETReqsDB2Copilot.hs
@@ -94,10 +94,12 @@ fret2CopilotModule' prefs smvSpec cocoSpec = unlines $ concat sections
                               else SMV.boolSpecNames smvSpec
 
     sections | fretReqsDB2CopilotUseCoCoSpec prefs
-             = [ imports, propDef, externs, clock, ftp, undef, spec, main' ]
+             = [ imports, propDef, externs, clock, ftp, undef, tpre, spec
+               , main'
+               ]
 
              | otherwise
-             = [ imports, propDef, externs, clock, ftp, spec, main' ]
+             = [ imports, propDef, externs, clock, ftp, tpre, spec, main' ]
 
     imports :: [String]
     imports =
@@ -145,6 +147,11 @@ fret2CopilotModule' prefs smvSpec cocoSpec = unlines $ concat sections
                , "pre = undefined"
                ]
 
+    tpre     = [ ""
+               , "tpre :: Stream Bool -> Stream Bool"
+               , "tpre = ([True] ++)"
+               ]
+
     spec     = [ ""
                , "-- | Complete specification. Calls the C function void handler(); when"
                , "-- the property is violated."
diff --git a/ogma-language-cocospec/CHANGELOG.md b/ogma-language-cocospec/CHANGELOG.md
index 07db695..8e67239 100644
--- a/ogma-language-cocospec/CHANGELOG.md
+++ b/ogma-language-cocospec/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Revision history for ogma-language-cocospec
 
+## [1.X.Y] - 2023-05-21
+
+* Add unary operators ZtoPre, YtoPre (#86).
+
 ## [1.0.8] - 2023-03-21
 
 * Version bump 1.0.8 (#81).
diff --git a/ogma-language-cocospec/grammar/CoCoSpec.cf b/ogma-language-cocospec/grammar/CoCoSpec.cf
index be38390..944ce52 100644
--- a/ogma-language-cocospec/grammar/CoCoSpec.cf
+++ b/ogma-language-cocospec/grammar/CoCoSpec.cf
@@ -49,12 +49,14 @@ BoolSpecOp2ST.  BoolSpec ::= "ST" "(" NumExpr "," NumExpr "," BoolSpec "," BoolS
 
 -- Boolean Operators
 
-Op1Once.   Op1Pre ::= "O"   ;
-Op1Pre.    Op1Pre ::= "pre" ;
-Op1Hist.   Op1Pre ::= "H"   ;
-Op1Y.      Op1Pre ::= "Y"   ;
-Op1Not.    Op1Pre ::= "not" ;
-Op1Bang.   Op1Pre ::= "!"   ;
+Op1Once.   Op1Pre ::= "O"      ;
+Op1Pre.    Op1Pre ::= "pre"    ;
+Op1YtoPre. Op1Pre ::= "YtoPre" ;
+Op1ZtoPre. Op1Pre ::= "ZtoPre" ;
+Op1Hist.   Op1Pre ::= "H"      ;
+Op1Y.      Op1Pre ::= "Y"      ;
+Op1Not.    Op1Pre ::= "not"    ;
+Op1Bang.   Op1Pre ::= "!"      ;
 
 Op2And.     Op2In ::= "and"     ;
 Op2Amp.     Op2In ::= "&"       ;