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