"If a bird is an ostrich, then it cannot fly." If a bird can fly, then it is not an ostrich. If a bird is not an ostrich, then it can fly. If a bird cannot fly, then it is an ostrich. If a bird is an ostrich, then it cannot fly.
"If a bird is an ostrich, then it cannot fly." If a bird can fly, then it is not an ostrich. If a bird is not an ostrich, then it can fly. If a bird cannot fly, then it is an ostrich. If a bird is an ostrich, then it cannot fly.
