def init_lambda_segment(G, lambda_function_doc):
    """Returns the Initialization subsegment id of a lambda function or None if warm start."""
    lambda_subsegments = G.successors(lambda_function_doc['id'])
    init_subsegments = (s for s in lambda_subsegments if G.nodes[s]['doc']['name'] == 'Initialization')  # noqa: E501
    return next(init_subsegments, None)