What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? David is not a bartender. David is a bartender if and only if Patricia is a taxi driver. Mary is a scientist if and only if William is a writer. Mary is not a scientist.
David is not a bartender and Mary is not a scientist. Mary is not a scientist and David is not a bartender. If David were a bartender, then Patricia is a taxi driver. If Mary were a scientist, then William is a writer. If Patricia were a taxi driver, then David is a bartender. If William were a writer, then Mary is a scientist. Patricia is not a taxi driver. William is not a writer.