def METHOD_NAME(graph: DataflowGraph) -> None:
    x: AssignableNodeReference = get_dataflow_node(graph, "x")
    y: AssignableNodeReference = get_dataflow_node(graph, "y")
    z: AssignableNodeReference = get_dataflow_node(graph, "z")
    x.assign(y, Statement(), graph)
    y.assign(z, Statement(), graph)
    leaves: Set[AssignableNode] = set(x.leaf_nodes())
    assert isinstance(z, DirectNodeReference)
    assert leaves == {z.node}