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_6, if x_6 were a doctor, then x_6 is a mechanic. For all x_6, if x_6 were a mechanic, then x_6 is a doctor. For all x_6, if x_6 were a mechanic, then x_6 is a doctor.
For all x_6, x_6 is a doctor if and only if x_6 is a mechanic can be inferred via the universal biconditional introduction rule. For all x_6, x_6 is a doctor if and only if x_6 is a mechanic can be inferred via the universal biconditional introduction rule. For all x_6, x_6 is a mechanic if and only if x_6 is a doctor can be inferred via the universal biconditional introduction rule. For all x_6, x_6 is a mechanic if and only if x_6 is a doctor can be inferred via the universal biconditional introduction rule.