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_11, x_11 is not fast. For all x_11, x_11 is a cashier or x_11 will write a letter. There is at least one x_11 for which x_11 is a cashier if and only if x_11 is a server. For all x_11, x_11 is fast if and only if x_11 is a server.
For all x_11, if x_11 were fast, then x_11 is a server can be inferred via the universal biconditional elimination rule. There is at least one x_11 for which if x_11 were a cashier, then x_11 is a server can be inferred via the existential biconditional elimination rule. For all x_11, if x_11 were a server, then x_11 is fast can be inferred via the universal biconditional elimination rule. There is at least one x_11 for which if x_11 were a server, then x_11 is a cashier can be inferred via the existential biconditional elimination rule. For all x_11, x_11 is not a server can be inferred via the universal biconditional elimination rule.