docs: public static void mult( double a[] , int offsetA ,
                               DMatrixSparseCSC B ,
                               double c[] , int offsetC )
      {
          if( a.length-offsetA < B.numRows)
              throw new IllegalArgumentException("Length of 'a' isn't long enough");
          if( c.length-offsetC < B.numCols)
              throw new IllegalArgumentException("Length of 'c' isn't long enough");
  
          for (int k = 0; k < B.numCols; k++) {
              int idx0 = B.col_idx[k  ];
              int idx1 = B.col_idx[k+1];
  
              double sum = 0;
              for (int indexB = idx0; indexB < idx1; indexB++) {
                  sum += a[offsetA+B.nz_rows[indexB]]*B.nz_values[indexB];
              }
              c[offsetC+k] = sum;
          }
      }