What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? David is playing tennis if and only if Mary is an artist. Robert is a doctor if and only if Mary is an artist. David is playing tennis.
If David plays tennis, then Mary is an artist. If Robert were a doctor, then Mary is an artist. If Mary were an artist, then David is playing tennis. If Mary were an artist, then Robert is a doctor.