docs: public static void solveTranL(DMatrixSparseCSC L , double []x )
      {
          final int N = L.numCols;
  
          for (int j = N-1; j >= 0; j--) {
              int idx0 = L.col_idx[j];
              int idx1 = L.col_idx[j+1];
  
              for (int p = idx0+1; p < idx1; p++) {
                  x[j] -= L.nz_values[p]*x[L.nz_rows[p]];
              }
              x[j] /= L.nz_values[idx0];
          }
      }