def _get_lcs_length_matrix(s1: str, s2: str) -> list:
    """Let m = len(s1) and n = len(s2), then this function returns a
    (m + 1)x(n + 1) matrix, specifically it returns a list of length m + 1,
    whose elements are lists of length (n + 1).

    The "+ 1" in (m + 1) and (n + 1) is because the first row and column are
    reserved for the cases where we compare with empty sequences."""
    return [[0 for _ in range(len(s2) + 1)] for _ in range(len(s1) + 1)]