code: public boolean remove     ( Object key,Object val ) {
      return Objects.equals(putIfMatch( key,TOMBSTONE, val ), val);
    }
docs: Atomically do a {@link #remove(Object)} if-and-only-if the key is mapped
  to a value which is <code>equals</code> to the given value.
  @throws NullPointerException if the specified key or value is null