public SelectExprEnumerator Init(SelectExprEnumerable <T, TR> parent, IPoolingEnumerator <T> src, Func <T, TR> mutator)
 {
     _src     = src;
     _parent  = parent;
     _mutator = mutator;
     return(this);
 }
 public void Dispose()
 {
     _parent?.Dispose();
     _parent = default;
     _src?.Dispose();
     _src = default;
     Pool <SelectExprEnumerator> .Return(this);
 }