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_13, x_13 is sad or x_13 will take a plane. There is at least one x_13 for which x_13 is sad if and only if x_13 will climb a mountain. For all x_13, if x_13 takes a plane, then x_13 is climbing a mountain.
There is at least one x_13 for which if x_13 were sad, then x_13 is climbing a mountain can be inferred via the existential biconditional elimination rule. There is at least one x_13 for which if x_13 climbs a mountain, then x_13 is sad can be inferred via the existential biconditional elimination rule.