What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? If David were sad, then Richard is a doctor. For all x_15, x_15 is not a doctor. David is sad if and only if it is sunny. If Mary were a writer, then it is sunny. If it is sunny, then Mary is a writer.
Mary is a writer if and only if it is sunny. It is sunny if and only if Mary is a writer. If David were sad, then it is sunny. If it is sunny, then David is sad.