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: Thomas is driving a car if and only if David is writing a letter. For all y, y will not write a letter. For all x_7, x_7 will work if and only if x_7 will drive a car.
If Thomas drives a car, then David is writing a letter can be inferred via the biconditional elimination rule. If David writes a letter, then Thomas is driving a car can be inferred via the biconditional elimination rule. For all x_7, if x_7 works, then x_7 is driving a car can be inferred via the universal biconditional elimination rule. For all x_7, if x_7 drives a car, then x_7 is working can be inferred via the universal biconditional elimination rule.