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: For all x, x will drive a car or x is not sad. There is at least one x for which x is sad if and only if x will write a letter.
For all x, if x were sad, then x is driving a car can be inferred via the universal rewriting rule. There is at least one x for which if x were sad, then x is writing a letter can be inferred via the existential biconditional elimination rule. There is at least one x for which if x writes a letter, then x is sad can be inferred via the existential biconditional elimination rule.