(pre-existing)

constify?
```
-        const Rlist *input_list = GetListReferenceArgument(ctx, fp, varname, NULL);
+        const Rlist* const input_list = GetListReferenceArgument(ctx, fp, varname, NULL);
```

Style?
```for (return_list = input_list; return_list && index--; return_list = return_list->next);```