What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? William is not a janitor. Robert is a soldier or William is not a janitor. Robert is a soldier if and only if William is a janitor.
If William were a janitor, then Robert is a soldier. If Robert were a soldier, then William is a janitor. If William were a janitor, then Robert is a soldier. Robert is not a soldier.