What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? If it is windy, then Barbara is driving a car. If Barbara drives a car, then it is windy. It is windy if and only if Barbara is writing a letter.
It is windy if and only if Barbara is driving a car. Barbara is driving a car if and only if it is windy. If it is windy, then Barbara is writing a letter. If Barbara writes a letter, then it is windy.