What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? There is at least one x_4 for which x_4 will not take a plane or x_4 is an astronaut. For all x_4, x_4 will take a plane. For all x_4, if x_4 writes a letter, then x_4 is sleeping. For all x_4, if x_4 were an astronaut, then x_4 is sleeping.
Nothing can be inferred from these premises.