(Adapted from @Barwise+Etchemendy:1993 .) Given the following, can you prove that the unicorn is mythical? How about magical? Horned?
If the unicorn is mythical, then it is immortal, but if it is not mythical, then it is a mortal mammal. If the unicorn is either immortal or a mammal, then it is horned. The unicorn is magical if it is horned.