checkArgument(incidentEdge.nodeU().equals(thisNode));

        N successor = incidentEdge.nodeV();
        V value = successorNodeToValueFn.apply(successor);

        Object existingValue = adjacentNodeValues.put(successor, value);
        if (existingValue != null) {
          checkArgument(existingValue == PRED);
          adjacentNodeValues.put(successor, new PredAndSucc(value));
        }

    }
  }

  private static <T extends @Nullable Object> void successor(AvlNode<T> a, AvlNode<T> b) {
    a.succ = b;
    b.pred = a;
  }

  private static <T extends @Nullable Object> void successor(
      AvlNode<T> a, AvlNode<T> b, AvlNode<T> c) {
    successor(a, b);
    successor(b, c);
  }

  /*

    values[size] = value;

    insertIntoTableKToV(size, keyHash);
    insertIntoTableVToK(size, valueHash);

    int successor =
        (predecessor == ENDPOINT) ? firstInInsertionOrder : nextInInsertionOrder[predecessor];
    setSucceeds(predecessor, size);
    setSucceeds(size, successor);
    size++;
    modCount++;
    return null;
  }

  /**

        assertThat(network.edgesConnecting(predecessor, node)).isNotEmpty();
      }

      for (N successor : sanityCheckSet(network.successors(node))) {
        assertThat(network.predecessors(successor)).contains(node);
        assertThat(network.edgesConnecting(node, successor)).isNotEmpty();
      }

      for (E incidentEdge : sanityCheckSet(network.incidentEdges(node))) {
        assertTrue(