diff --git a/flake.nix b/flake.nix
index c655d07..a56eecd 100644
--- a/flake.nix
+++ b/flake.nix
@@ -26,6 +26,7 @@
           gyehoek = final.haskell-nix.project' {
             src = ./.;
             compiler-nix-name = "ghc912";
+            configureArgs = "-f-doctest";
             modules = [({ pkgs, lib, ...}: {
               packages.gyehoek.components.tests.test.preCheck =
                 let
diff --git a/golden/exec/factorial/exec b/golden/exec/factorial/exec
index f17cfa4..5133d63 100644
--- a/golden/exec/factorial/exec
+++ b/golden/exec/factorial/exec
@@ -1,2 +1,2 @@
 ret > ExitSuccess
-out > 720
+out > 2432902008176640000
diff --git a/golden/exec/factorial/source.scm b/golden/exec/factorial/source.scm
index ccc9aad..ff18514 100644
--- a/golden/exec/factorial/source.scm
+++ b/golden/exec/factorial/source.scm
@@ -2,5 +2,4 @@
                 (if (zero? n)
                     1
                     (* n (fac (- n 1)))))))
-  ;; 20 is the greatest `n` for which n! ≤ maxBount @Int
   (fac 20))
diff --git a/gyehoek.cabal b/gyehoek.cabal
index a29f415..c247dcb 100644
--- a/gyehoek.cabal
+++ b/gyehoek.cabal
@@ -13,6 +13,11 @@ build-type:    Simple
 -- extra-doc-files:    CHANGELOG.md
 -- extra-source-files:
 
+flag doctest
+     description: enable the doctest suite
+     default: True
+     manual: True
+
 common ghcstuffs-dev
   ghc-options:
     -Wno-unused-matches -Wno-missing-signatures -Wno-typed-holes
@@ -156,11 +161,14 @@ test-suite test
   default-language:   GHC2024
 
 -- https://github.com/martijnbastiaan/doctest-parallel/pull/66
--- test-suite doctest
---   import:         ghcstuffs, ghcstuffs-dev
---   type:           exitcode-stdio-1.0
---   hs-source-dirs: test
---   main-is:        doctest.hs
---   build-depends:
---     , base
---     , doctest-parallel  >=0.1
+test-suite doctest
+  import:         ghcstuffs, ghcstuffs-dev
+  type:           exitcode-stdio-1.0
+  hs-source-dirs: test
+  build-depends: base
+  default-extensions: CPP
+  main-is:        doctest.hs
+  if flag(doctest)
+    build-depends: doctest-parallel  >=0.1
+  else
+    cpp-options: "-DGYEHOEK_NO_DOCTEST"
diff --git a/src/Gyehoek/Stack/Syntax.hs b/src/Gyehoek/Stack/Syntax.hs
index 79549e4..ddca7a1 100644
--- a/src/Gyehoek/Stack/Syntax.hs
+++ b/src/Gyehoek/Stack/Syntax.hs
@@ -107,7 +107,7 @@ instance S.DataIso Block where
 
 instance S.DatumIso Tail where
   datumIso = S.match
-    $ S.With (S.headTagged1' "tail-call!" S.datumIso S.datumIso >>>)
+    $ S.With (S.headTagged1' "tail-call" S.datumIso S.datumIso >>>)
     $ S.With (if_ >>>)
     $ S.End
     where
diff --git a/test/Gyehoek/Test/Stack/VM.hs b/test/Gyehoek/Test/Stack/VM.hs
index 857b8bd..b9923f3 100644
--- a/test/Gyehoek/Test/Stack/VM.hs
+++ b/test/Gyehoek/Test/Stack/VM.hs
@@ -16,23 +16,23 @@ test_root = testGroup "stack machine"
       evalsTo [ObjImm (ImmInt 3)] [stkP|
         (define ($main)
           (pop-cont! %ktail)
-          (tail-call! %ktail 3))
+          (tail-call %ktail 3))
       |]
   , testCase "return constant" do
       evalsTo [ObjImm (ImmInt 123)] [stkP|
         (define ($main)
-          (tail-call! $silly))
+          (tail-call $silly))
         (define ($silly)
           (pop-cont! %ktail)
-          (tail-call! %ktail 123))
+          (tail-call %ktail 123))
       |]
   , testCase "identity function" do
       evalsTo [ObjImm (ImmInt 45)] [stkP|
         (define ($main)
-          (tail-call! $id 45))
+          (tail-call $id 45))
         (define ($id %x)
           (pop-cont! %ktail)
-          (tail-call! %ktail %x))
+          (tail-call %ktail %x))
       |]
   -- , testCase "square" do
   --     evalsTo [ObjImm (ImmInt 16)] [stkP|
@@ -41,11 +41,11 @@ test_root = testGroup "stack machine"
   , testCase "square" do
       evalsTo [ObjImm (ImmInt 16)] [stkP|
         (define ($main)
-          (tail-call! $square 4))
+          (tail-call $square 4))
         (define ($square %x)
           (prim %x2 (* %x %x))
           (pop-cont! %ktail)
-          (tail-call! %ktail %x2))
+          (tail-call %ktail %x2))
       |]
   , testCase "factorial" do
       let hsfac (n :: Int) = foldr (*) (1) [1..n]
@@ -54,18 +54,18 @@ test_root = testGroup "stack machine"
               (prim %x0 (zero? %n))
               (if %x0
                 (then (pop-cont! %ktail)
-                      (tail-call! %ktail 1))
+                      (tail-call %ktail 1))
                 (else (push! %n)
                       (prim %x1 (- %n 1))
                       (push-cont! $fac-k0)
-                      (tail-call! $fac %x1))))
+                      (tail-call $fac %x1))))
             (define ($fac-k0 %x2)
               (pop! %n)
               (prim %x3 (* %x2 %n))
               (pop-cont! %ktail)
-              (tail-call! %ktail %x3))
+              (tail-call %ktail %x3))
             (define ($main)
-              (tail-call! $fac #{n}))
+              (tail-call $fac #{n}))
           |]
       evalsTo [ObjImm (ImmInt 1)] $ fac 0
       evalsTo [ObjImm (ImmInt 1)] $ fac 1
@@ -73,32 +73,3 @@ test_root = testGroup "stack machine"
       -- 20 is the greatest `n` for which n! ≤ maxBount @Int
       evalsTo [ObjImm (ImmInt 2432902008176640000)] $ fac 20
   ]
-
---   ]
-
--- prims = testGroup "prims"
---   [ arith
---   , testCase "zero?" do
---       trivialPrimTest [ObjImm (ImmBool True)] $
---         PrimZeroP $ ValImm $ ImmInt 0
---       trivialPrimTest [ObjImm (ImmBool False)] $
---         PrimZeroP $ ValImm $ ImmInt 12
---   ]
-
--- trivialPrimTest rs p =
---   evalsTo rs
---   [ MkRoutine "main" []
---     [ PopCont "ktail"
---     , Prim "x1" p
---     , Call (ValReg "ktail") [ValReg "x1"]
---     ]
---   ]
-
--- arith = testGroup "arith"
---   [ testCase "multipy" do
---       trivialPrimTest [ObjImm (ImmInt 12)]
---         (PrimMul (ValImm $ ImmInt 3) (ValImm $ ImmInt 4))
---   , testCase "subtract" do
---       trivialPrimTest [ObjImm (ImmInt 14)]
---         (PrimSub (ValImm $ ImmInt 20) (ValImm $ ImmInt 6))
---   ]
diff --git a/test/doctest.hs b/test/doctest.hs
index 625343f..ba3125d 100644
--- a/test/doctest.hs
+++ b/test/doctest.hs
@@ -1,10 +1,16 @@
-{-# LANGUAGE DoAndIfThenElse #-}
 module Main where
 
+#ifndef GYEHOEK_NO_DOCTEST
 import Test.DocTest (mainFromCabal)
-import System.Environment (getArgs, lookupEnv)
+#endif
+import System.Environment (getArgs)
 import System.IO (stderr, hPutStrLn)
 
 
 main :: IO ()
-main = mainFromCabal "gyehoek" =<< getArgs
+main =
+#ifndef GYEHOEK_NO_DOCTEST
+  mainFromCabal "gyehoek" =<< getArgs
+#else
+  hPutStrLn stderr "skipping doctests due to broken nix environment."
+#endif