Added 'ambiguity_is_error' flag, which, if set, makes the parser fail,
rather than just issue a warning, when the input parsed is ambiguous.
theory ToyList = PreList:
datatype 'a list = Nil ("[]")
| Cons 'a "'a list" (infixr "#" 65)
consts app :: "'a list => 'a list => 'a list" (infixr "@" 65)
rev :: "'a list => 'a list"
primrec
"[] @ ys = ys"
"(x # xs) @ ys = x # (xs @ ys)"
primrec
"rev [] = []"
"rev (x # xs) = (rev xs) @ (x # [])"