.
.
.

;type expr = Ast.expr

;module type Reader = sig
  type ast
  ;value reader_expr: string -> ast -> expr -> expr -> expr
;end

;module Make (Read: Reader) = struct

  ;type ast = [= Read.ast
    | `Lid of string
    | `Seq of ast & ast
    | `Binding of ast & string
    | `Absent  of ast
    | `Present of ast
    | `List0   of ast
    | `Opt     of ast
    | `Test    of expr
    | `Alts  of ast list ]
.
.
.
=>
Error: The type Read.ast is not a polymorphic variant type