def split_pair_insertion_rules(rule: str) -> tuple[str, list[str]]:
    """Split pair insertion rule of the form 'XY -> Z' and return in the form 'XY', ['XZ', 'ZY']."""

    mapping = rule.split(" -> ")

    return mapping[0], [mapping[0][0] + mapping[1], mapping[1] + mapping[0][1]]