What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Mary is not a janitor. Barbara is playing a game. Mary is a janitor if and only if David is climbing a mountain.
Mary is not a janitor and Barbara is playing a game. Barbara is playing a game and Mary is not a janitor. If Mary were a janitor, then David is climbing a mountain. If David climbs a mountain, then Mary is a janitor. David is not climbing a mountain.