docs: public T extractMatrix(int y0 , int y1, int x0 , int x1 ) {
          if( y0 == SimpleMatrix.END ) y0 = mat.getNumRows();
          if( y1 == SimpleMatrix.END ) y1 = mat.getNumRows();
          if( x0 == SimpleMatrix.END ) x0 = mat.getNumCols();
          if( x1 == SimpleMatrix.END ) x1 = mat.getNumCols();
  
          T ret = createMatrix(y1-y0,x1-x0, mat.getType());
  
          ops.extract(mat, y0, y1, x0, x1, ret.mat, 0, 0);
  
          return ret;
      }