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_1, if x_1 were sad, then x_1 is a musician. For all x_1, if x_1 were a musician, then x_1 is sad.
For all x_1, x_1 is sad if and only if x_1 is a musician can be inferred via the universal biconditional introduction rule. For all x_1, x_1 is a musician if and only if x_1 is sad can be inferred via the universal biconditional introduction rule.