What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Mary is a janitor and David is an electrician. Robert is playing tennis if and only if it is overcast. It is not overcast.
Mary is a janitor. David is an electrician. If Robert plays tennis, then it is overcast. If it is overcast, then Robert is playing tennis. Robert is not playing tennis.