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