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 cashier, then x_3 is listening to a song. For all x_3, if x_3 listens to a song, then x_3 is a cashier. William is not a cashier.
For all x_3, x_3 is a cashier if and only if x_3 will listen to a song can be inferred via the universal biconditional introduction rule. For all x_3, x_3 will listen to a song if and only if x_3 is a cashier can be inferred via the universal biconditional introduction rule.