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: Patricia is writing a letter if and only if it is late. If Patricia writes a letter, then it is raining. Sarah is fast. If it rains, then Sarah is not fast.
It is not raining can be inferred via the modus tollens rule. If Patricia writes a letter, then it is late can be inferred via the biconditional elimination rule. If it is late, then Patricia is writing a letter can be inferred via the biconditional elimination rule.