def _tgrep_node_label_use_action(_s, _l, tokens):
    assert len(tokens) == 1
    assert tokens[0].startswith('=')
    return tokens[0][1:]
Returns the node label used to begin a tgrep_expr_labeled.  See
dataset/ETHPy150Open nltk/nltk/nltk/tgrep.py/_tgrep_node_label_use_action