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: If Richard writes a letter, then Richard is listening to a song. If Richard listens to a song, then Richard is writing a letter. If Richard writes a letter, then Richard is listening to a song.
Richard is writing a letter if and only if Richard is listening to a song can be inferred via the biconditional introduction rule. Richard is listening to a song if and only if Richard is writing a letter can be inferred via the biconditional introduction rule. Richard is listening to a song if and only if Richard is writing a letter can be inferred via the biconditional introduction rule. Richard is writing a letter if and only if Richard is listening to a song can be inferred via the biconditional introduction rule.