docs: private static <K, V> Entry<K, V> constrainedEntry(
        final Entry<K, V> entry, final MapConstraint<? super K, ? super V> constraint) {
      checkNotNull(entry);
      checkNotNull(constraint);
      return new ForwardingMapEntry<K, V>() {
        @Override
        protected Entry<K, V> delegate() {
          return entry;
        }
  
        @Override
        public V setValue(V value) {
          constraint.checkKeyValue(getKey(), value);
          return entry.setValue(value);
        }
      };
    }