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_15, if x_15 were a mechanic, then x_15 is not sad. There is at least one x_15 for which x_15 is a mechanic if and only if x_15 is a janitor. For all x_15, x_15 is a janitor or x_15 will not play squash. For all x_15, if x_15 were a janitor, then x_15 is playing squash. For all x_15, x_15 is sad.
For all x_15, x_15 is not a mechanic can be inferred via the universal modus tollens rule. For all x_15, if x_15 plays squash, then x_15 is a janitor can be inferred via the universal rewriting rule. There is at least one x_15 for which if x_15 were a mechanic, then x_15 is a janitor can be inferred via the existential biconditional elimination rule. There is at least one x_15 for which if x_15 were a janitor, then x_15 is a mechanic can be inferred via the existential biconditional elimination rule.