def _antnums_to_blpair(antnums):
    """
    Convert nested tuple of antenna numbers to baseline-pair integer.
    A baseline-pair integer is an i12 integer that is the antenna numbers
    + 100 directly concatenated (i.e. string contatenation).
    Ex: ((1, 2), (3, 4)) --> 101 + 102 + 103 + 104 --> 101102103014.

    Parameters
    ----------
    antnums : tuple
        nested tuple containing integer antenna numbers for a baseline-pair.
        Ex. ((ant1, ant2), (ant3, ant4))

    Returns
    -------
    blpair : <i12 integer
        baseline-pair integer
    """
    # get antennas
    ant1 = antnums[0][0] + 100
    ant2 = antnums[0][1] + 100
    ant3 = antnums[1][0] + 100
    ant4 = antnums[1][1] + 100

    # form blpair
    blpair = int(ant1*1e9 + ant2*1e6 + ant3*1e3 + ant4)

    return blpair