fix String_with_vars.sexp_of_t (#595)

src/string_with_vars.ml

@@ -85,8 +85,7 @@ let sexp_of_item =
     | Var (vs, s) -> List [sexp_of_var_syntax vs ;
                            Sexp.atom_or_quoted_string s]
 
-let sexp_of_t t = Sexp.To_sexp.list sexp_of_item t.items
-
+let sexp_of_ast t = Sexp.To_sexp.list sexp_of_item t.items
 
 let fold t ~init ~f =
   List.fold_left t.items ~init ~f:(fun acc item ->
@@ -197,3 +196,5 @@ let to_string t =
       | Text s -> s
       | Var (syntax, v) -> string_of_var syntax v)
     |> String.concat ~sep:""
+
+let sexp_of_t t = Sexp.To_sexp.string (to_string t)

src/string_with_vars.mli

@@ -18,6 +18,10 @@ val loc : t -> Loc.t
 
 val sexp_of_t : t -> Sexp.t
 
+(** Same as [sexp_of_t] but the S-expression encodes the internal
+    structure of [t]. *)
+val sexp_of_ast : t -> Sexp.t
+
 val to_string : t -> string
 
 (** [t] generated by the OCaml code. The first argument should be