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 y, if y were a cashier, then y is writing a letter. For all y, if y writes a letter, then y is a cashier. Robert is a cashier.
There is at least one x for which x is a cashier can be inferred via the existential generalization rule. For all y, y is a cashier if and only if y will write a letter can be inferred via the universal biconditional introduction rule. For all y, y will write a letter if and only if y is a cashier can be inferred via the universal biconditional introduction rule.