- New argument `include_depth` on `preprocess_str()`.
- Existing function `preprocess()` is now a wrapper for private function
`preprocess_inner()` which uses `include_depth`.
- Commented arguments on all uses of `preprocess()` and `preprocess_str()`.
- Test `include_recursive` is fixed and not ignored.
- Without recursion limit, stack overflow occurs which crashes svls.
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