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_9, if x_9 plays squash, then x_9 is working. For all x_9, x_9 will work or x_9 will write a letter. There is at least one x_9 for which x_9 will not play squash. For all x_9, if x_9 works, then x_9 is playing squash.
There is at least one x_9 for which x_9 will not work can be inferred via the existential modus tollens rule. For all x_9, x_9 will play squash if and only if x_9 will work can be inferred via the universal biconditional introduction rule. For all x_9, x_9 will work if and only if x_9 will play squash can be inferred via the universal biconditional introduction rule.