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, x will take a plane if and only if x will play squash. There is at least one x for which if x were sad, then x is taking a plane. For all x, x is a lawyer. For all x, if x were a lawyer, then x is playing squash.
For all x, x will play squash can be inferred via the universal modus ponens rule. For all x, if x takes a plane, then x is playing squash can be inferred via the universal biconditional elimination rule. For all x, if x plays squash, then x is taking a plane can be inferred via the universal biconditional elimination rule.