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 it is late, then Barbara is a server. If Elisabeth cooks, then it is late. If Elisabeth cooks, then Barbara is a server. If Barbara were a server, then Elisabeth is cooking.
If it is late, then Elisabeth is cooking can be inferred via the transitivity rule. If Elisabeth cooks, then Barbara is a server can be inferred via the transitivity rule. If Barbara were a server, then it is late can be inferred via the transitivity rule. Elisabeth is cooking if and only if Barbara is a server can be inferred via the biconditional introduction rule. Barbara is a server if and only if Elisabeth is cooking can be inferred via the biconditional introduction rule.