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: Barbara is writing a letter if and only if David is not playing squash. James is not taking a plane. David is playing squash if and only if James is taking a plane. It is cold.
James is not taking a plane and it is cold can be inferred via the conjunction rule. It is cold and James is not taking a plane can be inferred via the conjunction rule. If Barbara writes a letter, then David is not playing squash can be inferred via the biconditional elimination rule. If David plays squash, then James is taking a plane can be inferred via the biconditional elimination rule. If David doesn't play squash, then Barbara is writing a letter can be inferred via the biconditional elimination rule. If James takes a plane, then David is playing squash can be inferred via the biconditional elimination rule. David is not playing squash can be inferred via the biconditional elimination rule.