def mergeUnitCompare(a, b):
    """
    _mergeUnitCompare_

    Compare two merge units.  They will be sorted first by run ID and then by
    lumi ID.
    """
    if a['run'] > b['run']:
        return 1
    elif a['run'] == b['run']:
        if a['lumi'] > b['lumi']:
            return 1
        elif a['lumi'] == b['lumi']:
            return 0
        else:
            return -1
    else:
        return -1
