What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? For all y, if y were a lawyer, then y is listening to a song. If Susan writes a letter, then Joseph is a lawyer. Charles is a janitor if and only if Joseph is listening to a song. If Charles were a janitor, then Susan is writing a letter.
If Charles were a janitor, then Joseph is a lawyer. If Charles were a janitor, then Joseph is listening to a song. If Joseph listens to a song, then Charles is a janitor.