def _normalize_protease_rules(rule):
    """
    Given a rule like: "AB, C; D, EF, G" convert to:
        ["", "", "AB", C",    "D", "EF", "G", ""]
    """
    n_side, c_side = rule.split(";")
    n_side = [i.strip() for i in n_side.split(",")]
    c_side = [i.strip() for i in c_side.split(",")]
    eight_tuple = [""] * (4 - len(n_side)) + n_side + c_side + [""] * (4 - len(c_side))
    assert len(eight_tuple) == 8
    return eight_tuple