def is_sub_seq(full_seq, sub_seq):
    """Return true if sub_seq is a sub-sequence of full_seq.

    """
    if len(sub_seq) == 0:
        return True
    for idx0,x0 in enumerate(full_seq):
        if x0 == sub_seq[0]:
            if is_sub_seq(full_seq[idx0+1:], sub_seq[1:]):
                return True
    return False