def is_connected(self):
        raise NotImplementedError()
Find the index of a child by identity, not value. Avoids issues with
dataset/ETHPy150Open EricssonResearch/calvin-base/calvin/runtime/south/plugins/transports/base_transport.py/BaseTransport.is_connected--crossover with--dataset/ETHPy150Open socketubs/pyhn/pyhn/lib/bs4_py3/element.py/Tag.index