public SelectManyExprEnumerator Init(
     SelectManyExprEnumerable <T, TR> parent,
     IPoolingEnumerator <T> src,
     Func <T, IPoolingEnumerable <TR> > mutator)
 {
     _src               = src;
     _finished          = false;
     _parent            = parent;
     _mutator           = mutator;
     _currentEnumerator = default;
     return(this);
 }
            public void Dispose()
            {
                _currentEnumerator?.Dispose();
                _currentEnumerator = default;

                _parent?.Dispose();
                _parent = default;

                _src.Dispose();
                _src = default;

                Pool <SelectManyExprEnumerator> .Return(this);
            }