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: Linda is making tea if and only if Robert is playing a game. Charles is not happy. If Robert plays a game, then Charles is happy.
Robert is not playing a game can be inferred via the modus tollens rule. If Linda makes tea, then Robert is playing a game can be inferred via the biconditional elimination rule. If Robert plays a game, then Linda is making tea can be inferred via the biconditional elimination rule.