2 fixes on syntax extension and dataframes

- fixed generated code for the syntax extension (type variable was
  not positionned correctly)
- changed subsetting operator for dataframes and listings (the former
  would give incorrect results)

src/base/rbase.ml

@@ -104,7 +104,7 @@ end
 
 let listing r = new listing r
 
-let subset2 = R.symbol ~generic: true "[[.data.frame"
+let subset2 = R.symbol ~generic: true ".subset2"
 
 class ['a] dataframe r = object (self)
   inherit ['a] listing r

src/syntax/pa_r.ml

@@ -76,7 +76,7 @@ module Make (Syntax : Sig.Camlp4Syntax) = struct
     let obj_type = fresh_type _loc in
     let constr =
       let y = random_var () in
-      <:expr< fun () -> let _ : Rbase.compound $obj_type$ = $lid:x$#compound in 
+      <:expr< fun () -> let (_ : Rbase.compound (< .. > as $obj_type$)) = $lid:x$#compound in 
                         fun ($lid:y$ : $obj_type$) -> ($lid:y$#$m$ : $comp_type$) >>
     in
     <:expr< let $lid:x$ = $e$ in