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, if x_3 were a politician, then x_3 is a server. For all x_3, if x_3 were a server, then x_3 is a politician. For all x_3, if x_3 were a politician, then x_3 is a mechanic. There is at least one x_3 for which x_3 is not a mechanic.
There is at least one x_3 for which x_3 is not a politician can be inferred via the existential modus tollens rule. For all x_3, if x_3 were a server, then x_3 is a mechanic can be inferred via the universal transitivity rule. For all x_3, x_3 is a politician if and only if x_3 is a server can be inferred via the universal biconditional introduction rule. For all x_3, x_3 is a server if and only if x_3 is a politician can be inferred via the universal biconditional introduction rule.