def METHOD_NAME():
    from sympy.abc import phi
    f = Box('f', Ob('x'), Ob('y'), data=[phi])
    g = Box('g', Ob('x'), Ob('y'), data=[phi])
    assert (f + g).free_symbols == {phi}
    assert (f + g).lambdify(phi)(1) == f.lambdify(phi)(1) + g.lambdify(phi)(1)
    empty_sum = Sum((), Ob('x'), Ob('y'))
    assert empty_sum.lambdify(phi)(123) == empty_sum
    assert empty_sum.subs(phi, 0) == empty_sum