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 Mary writes a letter, then David is playing a game. If Karen drinks water, then David is playing a game. Mary is not writing a letter. If David plays a game, then Mary is writing a letter.
David is not playing a game can be inferred via the modus tollens rule. Mary is writing a letter if and only if David is playing a game can be inferred via the biconditional introduction rule. David is playing a game if and only if Mary is writing a letter can be inferred via the biconditional introduction rule.