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: Charles is playing squash if and only if Susan is sad. For all x_8, x_8 will listen to a song or x_8 will play squash. If Charles listens to a song, then Susan is sad.
If Charles plays squash, then Susan is sad can be inferred via the biconditional elimination rule. If Susan were sad, then Charles is playing squash can be inferred via the biconditional elimination rule.