docs: public void _solveVectorInternal( double []vv )
      {
          // Solve L*Y = B
          int ii = 0;
  
          for( int i = 0; i < n; i++ ) {
              int ip = indx[i];
              double sum = vv[ip];
              vv[ip] = vv[i];
              if( ii != 0 ) {
  //                for( int j = ii-1; j < i; j++ )
  //                    sum -= dataLU[i* n +j]*vv[j];
                  int index = i*n + ii-1;
                  for( int j = ii-1; j < i; j++ )
                      sum -= dataLU[index++]*vv[j];
              } else if( sum != 0.0 ) {
                  ii=i+1;
              }
              vv[i] = sum;
          }
  
          // Solve U*X = Y;
          TriangularSolver_DDRM.solveU(dataLU,vv,n);
      }