new_contents: from sympy.matrices.expressions import MatrixSymbol, MatAdd
  
  X = MatrixSymbol('X', 2, 2)
  Y = MatrixSymbol('Y', 2, 2)
  
  def test_sort_key():
      assert MatAdd(Y, X).doit().args == (X, Y)
  
subject: add test for matadd sortkey