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_4, if x_4 were a taxi driver, then x_4 is running. For all x_4, if x_4 goes running, then x_4 is a taxi driver. There is at least one x_4 for which if x_4 were a taxi driver, then x_4 is running.
For all x_4, x_4 is a taxi driver if and only if x_4 will go running can be inferred via the universal biconditional introduction rule. For all x_4, x_4 will go running if and only if x_4 is a taxi driver can be inferred via the universal biconditional introduction rule. There is at least one x_4 for which x_4 is a taxi driver if and only if x_4 will go running can be inferred via the existential biconditional introduction rule. There is at least one x_4 for which x_4 will go running if and only if x_4 is a taxi driver can be inferred via the existential biconditional introduction rule.