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 sunny or Elisabeth is writing a letter. If it is cloudy, then Richard is not a cashier. Elisabeth is writing a letter if and only if Richard is a cashier. If it is sunny, then Richard is a cashier.
If Elisabeth writes a letter, then Richard is a cashier can be inferred via the biconditional elimination rule. If Richard were a cashier, then Elisabeth is writing a letter can be inferred via the biconditional elimination rule.