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_4, x_4 is a janitor if and only if x_4 is a soldier. For all x_4, if x_4 were a soldier, then x_4 is a janitor. There is at least one x_4 for which x_4 is not a soldier.
For all x_4, if x_4 were a janitor, then x_4 is a soldier can be inferred via the universal biconditional elimination rule. For all x_4, if x_4 were a soldier, then x_4 is a janitor can be inferred via the universal biconditional elimination rule. There is at least one x_4 for which x_4 is not a janitor can be inferred via the existential biconditional elimination rule.