if (!exList.contains(e)) {
            exList.add(e);
        }

        List<MetadataGraphEdge> inList = incidentEdges.computeIfAbsent(vTo, k -> new ArrayList<>());

        if (!inList.contains(e)) {
            inList.add(e);
        }

        return this;
    }
    // ------------------------------------------------------------------------

   * created arrays to have a {@code @Nullable} element type even when they're created directly with
   * {@code new ImmutableMapEntry[...]}, so it seems silly to insist on that only here.
   */
  @SuppressWarnings("unchecked") // Safe as long as the javadocs are followed
  static <K, V> ImmutableMapEntry<K, V>[] createEntryArray(int size) {