At any time I am either busy, procrastinating, or working on my my homework.
All afternoon I was not busy.
All afternoon I was not working on my homework.
Therefore all afternoon I was procrastinating.
This is a sound syllogism, being both true and valid. Its also disjunctive, following the format of
A is B, C, or D.
A is not B.
A is not D.
Therefore A is C.