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_15, if x_15 were sad, then x_15 is climbing a mountain. There is at least one x_15 for which x_15 will play tennis if and only if x_15 is a server. For all x_15, x_15 is sad or x_15 is not a server. For all x_15, if x_15 climbs a mountain, then x_15 is playing tennis.
For all x_15, if x_15 were sad, then x_15 is playing tennis can be inferred via the universal transitivity rule. For all x_15, if x_15 were a server, then x_15 is sad can be inferred via the universal rewriting rule. There is at least one x_15 for which if x_15 plays tennis, then x_15 is a server can be inferred via the existential biconditional elimination rule. There is at least one x_15 for which if x_15 were a server, then x_15 is playing tennis can be inferred via the existential biconditional elimination rule.