minor modification in let binding syntax

- patterns used after a let binding must be enclosed in parenthesis
  consider : type t( 'a ) = [ 'a* ] ;;
  this function declaration works
    let (t(Int) & x) = [ 1 ] ;;
  while without parenthesis will raise a failure.

parser/parser.ml

@@ -94,6 +94,7 @@ let is_fun_decl =
      (fun strm ->
        match Stream.npeek 3 strm with
 	 | [ KEYWORD "fun", _; IDENT _, _; KEYWORD "(", _ ]
+       | [ PTYPE _, _; _ ; _ ]
 	 | [ IDENT _, _; KEYWORD "(", _; _ ] -> ()
 	 | _ -> raise Stream.Failure
      )
@@ -490,8 +491,8 @@ EXTEND Gram
 
 
  fun_decl: [
-   [ f = OPT located_ident; "("; (a,b) = fun_decl_after_lparen ->
-       (f,a,b)
+   [ f = OPT located_ident; "("; (a,b) = fun_decl_after_lparen -> (f,a,b)
+   | f = PTYPE; (a,b) = fun_decl_after_lparen -> (Some (lop _loc,ident f),a,b)
    ]
  ];