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 a nurse if and only if Richard is writing a letter. For all x_10, x_10 is not a nurse.
If Barbara were a nurse, then Richard is writing a letter can be inferred via the biconditional elimination rule. If Richard writes a letter, then Barbara is a nurse can be inferred via the biconditional elimination rule.