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: Susan is writing a letter if and only if it is snowing. For all z, z will not write a letter.
If Susan writes a letter, then it is snowing can be inferred via the biconditional elimination rule. If it snows, then Susan is writing a letter can be inferred via the biconditional elimination rule.