@staticmethod
def merge_or_append_ems_to_pairs(ems_in_slot, pair_id, pairs):
    """if there is an existing ems_pair including the current mention: merge"""
    if pair_id is not None:
        pairs[pair_id].update(ems_in_slot)
    else:
        pairs.append(set(tuple(ems_in_slot)))
