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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_subnetwork(statements, nodes, relevance_network=None, relevance_node_lim=10):
 """Return a PySB model based on a subset of given INDRA Statements. Statements are first filtered for nodes in the given list and other nodes are optionally added based on relevance in a given network. The filtered statements are then assembled into an executable model using INDRA's PySB Assembler. Parameters statements : list[indra.statements.Statement] A list of INDRA Statements to extract a subnetwork from. nodes : list[str] The names of the nodes to extract the subnetwork for. relevance_network : Optional[str] The UUID of the NDEx network in which nodes relevant to the given nodes are found. relevance_node_lim : Optional[int] The maximal number of additional nodes to add to the subnetwork based on relevance. Returns ------- model : pysb.Model A PySB model object assembled using INDRA's PySB Assembler from the INDRA Statements corresponding to the subnetwork. """

    if relevance_network is not None:
        relevant_nodes = _find_relevant_nodes(nodes, relevance_network,
                                              relevance_node_lim)
        all_nodes = nodes + relevant_nodes
    else:
        all_nodes = nodes
    filtered_statements = _filter_statements(statements, all_nodes)
    pa = PysbAssembler()
    pa.add_statements(filtered_statements)
    model = pa.make_model()
    return model