def splitBy(s1, s2):
    """Split sequence s1 by sequence s2.
    Returns True and prefix + postfix, or just False and None None
    """
    if len(s2) == 0:
        return True, s1, ()
    
    for i in range(len(s1) - len(s2)+1):
        if s1[i:i+len(s2)] == s2:
            return True, s1[:i], s1[i+len(s2):]
    return False, None, None