'Returns lhs - rhs, but treats arguments like symbols, so things like
oo - oo return 0, instead of a nan.'

@staticmethod
def _combine_inverse(lhs, rhs):

    from sympy import oo, I, expand_mul
    if (((lhs == oo) and (rhs == oo)) or ((lhs == (oo * I)) and (rhs == (oo * I)))):
        return S.Zero
    return expand_mul((lhs - rhs))
