docs: public static void solveTranL( double L[] , double []b , int n )
      {
          for( int i =n-1; i>=0; i-- ) {
              double sum = b[i];
              for( int k = i+1; k <n; k++ ) {
                  sum -= L[k*n+i]* b[k];
              }
              b[i] = sum/L[i*n+i];
          }
      }