def remove_last_syllable(phon: str) -> str:
    """Remove the last syllable boundary in a phonetic string.

    Example: Turn `fa:-n` (stem of `fa:-n@`) into `fa:n`.
    """
    # Find the last hyphen marking a syllable boundary and remove it.
    i = phon.rfind("-")
    j = i + 1
    return phon[:i] + phon[j:]