code: public final void connectIfNotFound(N n1, E edge, N n2) {
      if (!isConnected(n1, edge, n2)) {
        connect(n1, edge, n2);
      }
    }
docs: Connects two nodes in the graph with an edge if such edge does not already
  exists between the nodes.
  
  @param n1 First node.
  @param edge The edge.
  @param n2 Second node.