# type _ complist =
  | One : ('a -> 'b) -> ('a * 'b) complist
  | Cons : ('a -> 'b) * ('b * 'c) complist -> ('a * 'c) complist;;
type _ complist =
    One : ('a -> 'b) -> ('a * 'b) complist
  | Cons : ('a -> 'b) * ('b * 'c) complist -> ('a * 'c) complist

# let rec comp_length = function
  | One _ -> 1
  | Cons (_, xs) -> 1 + comp_length xs;;
Error: This expression has type (ex#0 * 'a) complist
       but an expression was expected of type ('b * 'a) complist
       The type constructor ex#0 would escape its scope