docs: private Map<NodeKey, Integer> computeReferrersCountDelta( List<NodeKey> addedReferrers,
                                                                List<NodeKey> removedReferrers ) {
          Map<NodeKey, Integer> referrersCountDelta = new HashMap<NodeKey, Integer>(0);
  
          Set<NodeKey> addedReferrersUnique = new HashSet<NodeKey>(addedReferrers);
          for (NodeKey addedReferrer : addedReferrersUnique) {
              int referrersCount = Collections.frequency(addedReferrers, addedReferrer)
                                   - Collections.frequency(removedReferrers, addedReferrer);
              referrersCountDelta.put(addedReferrer, referrersCount);
          }
  
          Set<NodeKey> removedReferrersUnique = new HashSet<NodeKey>(removedReferrers);
          for (NodeKey removedReferrer : removedReferrersUnique) {
              // process what's left in the removed list, only if not found in the added
              if (!referrersCountDelta.containsKey(removedReferrer)) {
                  referrersCountDelta.put(removedReferrer, -1 * Collections.frequency(removedReferrers, removedReferrer));
              }
          }
          return referrersCountDelta;
      }