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_12, x_12 is a taxi driver if and only if x_12 will write a letter. For all x_12, if x_12 sleeps, then x_12 is writing a letter. There is at least one x_12 for which x_12 will play tennis. For all x_12, if x_12 were a doctor, then x_12 is writing a letter. For all x_12, x_12 is a doctor or x_12 will sleep.
For all x_12, x_12 will write a letter can be inferred via the universal disjunction elimination rule. For all x_12, if x_12 were a taxi driver, then x_12 is writing a letter can be inferred via the universal biconditional elimination rule. For all x_12, if x_12 writes a letter, then x_12 is a taxi driver can be inferred via the universal biconditional elimination rule.