public void Dispose()
            {
                _src?.Dispose();
                _src = null;

                _parent?.Dispose();
                _parent = default;

                Pool <UnionExprEnumerator> .Return(this);
            }
 public UnionExprEnumerator Init(UnionExprEnumerable <T> parent, IPoolingEnumerator <KeyValuePair <T, int> > src)
 {
     _src    = src;
     _parent = parent;
     return(this);
 }