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 server. For all z, if z were a server, then z is a sailor. For all z, if z were a sailor, then z is a server. Thomas is cooking.
David is not a server and Thomas is cooking. Thomas is cooking and David is not a server. For all z, z is a server if and only if z is a sailor. For all z, z is a sailor if and only if z is a server.