Expected result is: 2*(x + y)*Derivative(g(x**2 + y**2), x**2 + y**2)

changed to:
@XFAIL
def test_U9():
    ''' Maple syntax:
    O29 := diff(f(x, y), x) + diff(f(x, y), y);
                          /d         \   /d         \
                          |-- f(x, y)| + |-- f(x, y)|
                          \dx        /   \dy        /
    
    O30 := factor(subs(f(x, y) = g(x^2 + y^2), %));
                                    2    2
                            2 D(g)(x  + y ) (x + y)
    '''
    x,y = symbols('x y', real=True)
    su = diff(f(x, y), x) + diff(f(x, y), y)
    s2 = Subs(su,f(x,y),g(x**2+y**2)).doit()
    s3 = s2.doit().factor() 
    # Subs not performmed, s3 = 2*(x + y)*Subs(Derivative(g(_xi_1), _xi_1), (_xi_1,), (x**2 + y**2,))
    assert s3 == 2*(x + y)*Derivative(g(x**2 + y**2), x**2 + y**2) # raises ValueError: Can't differentiate wrt the variable: x**2 + y**2, 1