>();      // Set all but the first element of the output tensor to 0.     const int N = input.size();     for (int i = 0; i < N; i++) {       output_flat(i) = 0;     }      // Preserve the requested input value     output_flat(preserve_index) = input(preserve_index);      // Preserve the first input value if possible.     if (N > 0) output_flat(0) = input(0);   } };  