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 David were an electrician, then Richard is excited. Richard is not excited. For all x_7, x_7 is a writer if and only if x_7 is an electrician.
David is not an electrician can be inferred via the modus tollens rule. For all x_7, if x_7 were a writer, then x_7 is an electrician can be inferred via the universal biconditional elimination rule. For all x_7, if x_7 were an electrician, then x_7 is a writer can be inferred via the universal biconditional elimination rule.