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: There is at least one y_4 for which if y_4 were fast, then y_4 is sad. For all y_4, y_4 is not sad. For all y_4, if y_4 were sad, then y_4 is fast.
There is at least one y_4 for which y_4 is not fast can be inferred via the existential modus tollens rule. There is at least one y_4 for which y_4 is fast if and only if y_4 is sad can be inferred via the existential biconditional introduction rule. There is at least one y_4 for which y_4 is sad if and only if y_4 is fast can be inferred via the existential biconditional introduction rule.