code: public int getExisting(Object key) {
          if (key == null) { key = UniqueTag.NULL_VALUE; }
          int index = findIndex(key);
          if (0 <= index) {
              return values[index];
          }
          // Key must exist
          Kit.codeBug();
          return 0;
      }
docs: Get integer value assigned with key.
  @return key integer value
  @throws RuntimeException if key does not exist