What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? John is not an astronaut. If John were an astronaut, then David is taking a plane. David is taking a plane if and only if John is an astronaut.
If David takes a plane, then John is an astronaut. If John were an astronaut, then David is taking a plane. David is not taking a plane.