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 it is windy, then William is writing a letter. Barbara is not an office clerk. It is windy or William is not writing a letter. William is writing a letter if and only if Barbara is an office clerk.
If William writes a letter, then it is windy can be inferred via the rewriting rule. If William writes a letter, then Barbara is an office clerk can be inferred via the biconditional elimination rule. If Barbara were an office clerk, then William is writing a letter can be inferred via the biconditional elimination rule. William is not writing a letter can be inferred via the biconditional elimination rule.