Fix datatype of expr with locations

CLAUDE.md

@@ -231,6 +231,7 @@ dune test
 2. Understand term unification before touching `unification.ml`
 3. AST changes require updates to parser, evaluator, and pretty-printer
 4. Test with existing examples in `examples/` after changes
+5. **Always run `dune fmt` after finishing code modifications** to ensure consistent formatting
 
 ### Important concepts for contributors:
 - **Polarity** drives interaction - positive/negative rays fuse

bin/sgen.ml

@@ -104,8 +104,9 @@ let watch input_file timeout =
 let preprocess_only input_file =
   let expr = parse input_file in
   let preprocessed = Expr.preprocess expr in
-  preprocessed |> List.map ~f:Expr.to_string |> String.concat ~sep:"\n"
-  |> Stdlib.print_endline
+  preprocessed
+  |> List.map ~f:(fun e -> Expr.to_string e.Expr.content)
+  |> String.concat ~sep:"\n" |> Stdlib.print_endline
 
 let input_file_arg =
   let doc = "Input file to process." in