android/guava/src/com/google/common/collect/ConcurrentHashMultiset.java
@Override
  Set<E> createElementSet() {
    final Set<E> delegate = countMap.keySet();
    return new ForwardingSet<E>() {
      @Override
      protected Set<E> delegate() {
        return delegate;
      }

      @Override
      public boolean contains(@NullableDecl Object object) {
        return object != null && Collections2.safeContains(delegate, object);
      }

      @Override
      public boolean containsAll(Collection<?> collection) {
        return standardContainsAll(collection);
      }

      @Override
      public boolean remove(Object object) {
        return object != null && Collections2.safeRemove(delegate, object);
      }

      @Override
      public boolean removeAll(Collection<?> c) {
        return standardRemoveAll(c);
      }
    };
  }
@Override
  Set<E> createElementSet() {
    final Set<E> delegate = countMap.keySet();
    return new ForwardingSet<E>() {
      @Override
      protected Set<E> delegate() {
        return delegate;
      }

      @Override
      public boolean contains(@NullableDecl Object object) {
        return object != null && Collections2.safeContains(delegate, object);
      }

      @Override
      public boolean containsAll(Collection<?> collection) {
        return standardContainsAll(collection);
      }

      @Override
      public boolean remove(Object object) {
        return object != null && Collections2.safeRemove(delegate, object);
      }

      @Override
      public boolean removeAll(Collection<?> c) {
        return standardRemoveAll(c);
      }
    };
  }
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