Fix input-output signatures (#999)

Since 0.101.0 we will finally catch more illegal `def` signatures. As
the grammar for input/output types is rather restricted, this would
error otherwise

modules/recursion/README.md

@@ -43,7 +43,7 @@ use tramp.nu
 # This version just returns either the value or a thunk.
 # Meant to be used in a trampoline
 # But still uses APS
-def fact [n: int, acc=1] -> int {
+def fact [n: int, acc=1]: nothing -> int {
   if $n <= 1 { return $acc } else {
     {|| fact ($n - 1) ($n * $acc) } # The thunk being returned to the trampoline
   }