The type constructor would escape its scope (or, what on earth is this error?)

type_constructor_escape_scope.ml

type _ value = Bool : bool -> bool value
	     | Int : int -> int value

type _ atom = Abool : bool -> bool atom
	    | Aint : int -> int atom

type _ sexp = Sexp_atom : 'a atom -> 'b atom sexp

let val_of_atom : type t. t atom -> t value =
  function
  | Abool b -> Bool b
  | Aint i -> Int i

let val_of_sexp = function
  | Sexp_atom a -> val_of_atom a


(* 

Error: This expression has type a#0 value
       but an expression was expected of type a#0 value
       The type constructor a#0 would escape its scope 

"This expression" being `val_of_atom a` in val_of_sexp

*)