def split(numbers: list, i) -> list:
    """If any regular number is 10 or greater, the leftmost such regular number splits.

    To split a regular number, replace it with a pair;
    the left element of the pair should be the regular number divided by two and rounded down,
    while the right element of the pair should be the regular number divided by two and rounded up.

    For example, 10 becomes [5,5], 11 becomes [5,6], 12 becomes [6,6], and so on.

    """
    assert numbers[i] >= 10
    numbers = (
        numbers[:i]
        + ["[", numbers[i] // 2, (numbers[i]+1) // 2, "]"]
        + numbers[i + 1 :]
    )
    return numbers