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: For all x_7, if x_7 were rich, then x_7 is not fast. For all x_7, x_7 is fast. Sarah is writing a letter if and only if Joseph is rich.
If Sarah writes a letter, then Joseph is rich can be inferred via the biconditional elimination rule. If Joseph were rich, then Sarah is writing a letter can be inferred via the biconditional elimination rule. For all x_7, x_7 is not rich can be inferred via the universal modus tollens rule.