Only the lonely are sad. Not all toads are ugly. Some geniuses are not able to tie their shoes. Some geniuses are not philosophers. Conversion consists in switching the subject and predicate terms.
All dogs are mammals.
No reptiles are humans. A term complement denotes a class complement. The complement of a class is the group consisting of everything that does not belong to that class. Thus the complement of the class of dogs is the group consisting of everything that is not a dog. But be careful.