if (elementSet == null) {
          elementSet = typePreservingSet(delegate().elementSet(), mutex);
        }
        return elementSet;
      }
    }

    @Override
    public Set<Multiset.Entry<E>> entrySet() {
      synchronized (mutex) {
        if (entrySet == null) {
          entrySet = typePreservingSet(delegate().entrySet(), mutex);
        }
        return entrySet;
      }

        if (elementSet == null) {
          elementSet = typePreservingSet(delegate().elementSet(), mutex);
        }
        return elementSet;
      }
    }

    @Override
    public Set<Multiset.Entry<E>> entrySet() {
      synchronized (mutex) {
        if (entrySet == null) {
          entrySet = typePreservingSet(delegate().entrySet(), mutex);
        }
        return entrySet;
      }