def get_mers(seq, size):
    """
    Get X-mers from seq.

    Example
    -------
    >>> get_mers('MEAIKHD', 3)
    {'MEA', 'EAI', 'AIK', 'KHD'}

    """
    mers = []
    mersa = mers.append
    for i in range(len(seq) - (size - 1)):
        mersa(seq[i:i + size])
    return set(mers)