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, x_4 is not a lawyer. There is at least one x_4 for which x_4 is a doctor or x_4 will listen to a song. For all x_4, x_4 is a doctor if and only if x_4 is a lawyer.
For all x_4, if x_4 were a doctor, then x_4 is a lawyer can be inferred via the universal biconditional elimination rule. For all x_4, if x_4 were a lawyer, then x_4 is a doctor can be inferred via the universal biconditional elimination rule. For all x_4, x_4 is not a doctor can be inferred via the universal biconditional elimination rule.