code: public Refs<T> addAll(Refs<T> add)
      {
          List<Ref<T>> overlap = new ArrayList<>();
          for (Map.Entry<T, Ref<T>> e : add.references.entrySet())
          {
              if (this.references.containsKey(e.getKey()))
                  overlap.add(e.getValue());
              else
                  this.references.put(e.getKey(), e.getValue());
          }
          add.references.clear();
          release(overlap);
          return this;
      }
docs: Merge two sets of references, ensuring only one reference is retained between the two sets