Function reverse_apply_hunks 

pub fn reverse_apply_hunks(
    current: &str,
    hunks: &[DiffHunk],
) -> Result<String, ReverseApplyError>

Reverses structured hunks against current content, returning the pre-image.

Each hunk’s new-side block must occur exactly once in current, and the matches must run in hunk order without overlap. Edits outside the matched spans are preserved; edits inside them refuse the whole file.

Requires

• hunks comes from split_unified_files (strict fidelity).

Ensures

• Ok(pre) replaces exactly the recorded new-side blocks with their old-side counterparts; Err computes nothing partial (pure).
• Total (terminating): bounded scans over current and hunks.