docs: public static SiblingCounter alter( final SiblingCounter counter,
                                          final int delta ) {
          assert counter != null;
          return new SiblingCounter() {
              @Override
              public int countSiblingsNamed( Name childName ) {
                  int count = counter.countSiblingsNamed(childName) + delta;
                  return count > 0 ? count : 0; // never negative
              }
          };
      }