guava/src/com/google/common/graph/ConfigurableMutableNetwork.java
@CanIgnoreReturnValue
  private NetworkConnections<N, E> addNodeInternal(N node) {
    NetworkConnections<N, E> connections = newConnections();
    checkState(nodeConnections.put(node, connections) == null);
    return connections;
  }
@CanIgnoreReturnValue
  private NetworkConnections<N, E> addNodeInternal(N node) {
    NetworkConnections<N, E> connections = newConnections();
    checkState(nodeConnections.put(node, connections) == null);
    return connections;
  }
Adds {@code node} to the graph and returns the associated {@link NetworkConnections}.

@throws IllegalStateException if {@code node} is already present