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_1, if y_1 were a writer, then y_1 is a cashier. For all y_1, if y_1 were a writer, then y_1 is a cashier. There is at least one y_1 for which if y_1 were a cashier, then y_1 is a writer.
There is at least one y_1 for which y_1 is a cashier if and only if y_1 is a writer can be inferred via the existential biconditional introduction rule. There is at least one y_1 for which y_1 is a cashier if and only if y_1 is a writer can be inferred via the existential biconditional introduction rule. There is at least one y_1 for which y_1 is a writer if and only if y_1 is a cashier can be inferred via the existential biconditional introduction rule. There is at least one y_1 for which y_1 is a writer if and only if y_1 is a cashier can be inferred via the existential biconditional introduction rule.