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_2, if x_2 were a mechanic, then x_2 is playing squash. For all x_2, x_2 will write a letter if and only if x_2 is poor. For all x_2, if x_2 were poor, then x_2 is a mechanic.
For all x_2, if x_2 were poor, then x_2 is playing squash can be inferred via the universal transitivity rule. For all x_2, if x_2 writes a letter, then x_2 is poor can be inferred via the universal biconditional elimination rule. For all x_2, if x_2 were poor, then x_2 is writing a letter can be inferred via the universal biconditional elimination rule.