Return list of groups in same order as in code2nt

def _init_grps(code2nt):
        """Return list of groups in same order as in code2nt"""
        seen = set()
        seen_add = seen.add
        groups = [nt.group for nt in code2nt.values()]
        return [g for g in groups if not (g in seen or seen_add(g))]