def _create_dirtree(a,chunksize=2):
    """create a directory tree from a single, long name

    e.g. "12345" --> ["1", "23", "45"]
    """
    b = a[::-1]  # reverse
    i = 0
    l = []
    while i < len(b):
        l.append(b[i:i+chunksize])
        i += chunksize
    return [e[::-1] for e in l[::-1]]