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 y_4, if y_4 climbs a mountain, then y_4 is a sailor. For all y_4, if y_4 were a sailor, then y_4 is climbing a mountain. For all y_4, y_4 is a musician or y_4 is a sailor. There is at least one y_4 for which if y_4 were a musician, then y_4 is climbing a mountain.
There is at least one y_4 for which if y_4 were a musician, then y_4 is a sailor can be inferred via the existential transitivity rule. There is at least one y_4 for which y_4 will climb a mountain can be inferred via the existential disjunction elimination rule. For all y_4, y_4 will climb a mountain if and only if y_4 is a sailor can be inferred via the universal biconditional introduction rule. For all y_4, y_4 is a sailor if and only if y_4 will climb a mountain can be inferred via the universal biconditional introduction rule.