What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: For all x_3, x_3 is an astronaut if and only if x_3 is a server. There is at least one x_3 for which x_3 is an astronaut or x_3 will write a letter. For all x_3, if x_3 writes a letter, then x_3 is a server.
For all x_3, if x_3 were an astronaut, then x_3 is a server can be inferred via the universal biconditional elimination rule. For all x_3, if x_3 were a server, then x_3 is an astronaut can be inferred via the universal biconditional elimination rule.