A law firm consists of five partners: Alice, Bob, Carol, Dave, and Ellen.
If Ellen is on a case, then Bob is not.
If Bob is on a case, then Carol is not.
If Carol is on a case, then Dave is.
If Dave is on a case, then Alice is not.
If Alice is on a case, then Ellen is not.
If only one partner can be on the case, who must it be?
