What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? David is a cashier if and only if Jessica is drinking water. David is writing a letter or it is not sunny. If David writes a letter, then David is a cashier.
If it is sunny, then David is writing a letter. If David were a cashier, then Jessica is drinking water. If Jessica drinks water, then David is a cashier.