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: It is cold or Sarah is not writing a letter. It is snowing if and only if Sarah is writing a letter.
If Sarah writes a letter, then it is cold can be inferred via the rewriting rule. If it snows, then Sarah is writing a letter can be inferred via the biconditional elimination rule. If Sarah writes a letter, then it is snowing can be inferred via the biconditional elimination rule.