Write an assertion that states that no duck can be both female and male
What problem(s) do you see if you tried to be more general (or more specific) than you were?
Write an assertion that states that nothing can be both a Tiger and a Dog
How can this be generalized to other animals? (i.e. one assertion that covers Tigers not being Dogs, and Horses not being Cows)
Does your representation allow something to be both a Tiger and a Mammal? If so, how? If not, maybe you should alter your representation so that it does!