def occurence(main_seq, sub_seq):
    """Check for the occurence of a sub sequence in a sequence

    >>>occurence(seq1,sub_seq1)

    """
    start = 0
    indices = []
    while True:
        start = main_seq.find(sub_seq, start)
        if start > 0:
            indices.append(start)
        else:
            break
        start += 1
    return indices
