'Checks that type A coerces to B, including subclasses.'

def check_coerce_to(self, A, B):

    self.assertCoerceTo(A, B)
    class SubclassOfA(A, ):
        pass
    self.assertCoerceTo(SubclassOfA, B)
    class SubclassOfB(B, ):
        pass
    self.assertCoerceTo(A, SubclassOfB)
    self.assertCoerceTo(SubclassOfA, SubclassOfB)
