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, if x_13 were fast, then x_13 is climbing a mountain. There is at least one x_13 for which x_13 will write a letter and x_13 will listen to a song. For all x_13, if x_13 goes running, then x_13 is fast. For all x_13, if x_13 writes a letter, then x_13 is climbing a mountain.
For all x_13, if x_13 goes running, then x_13 is climbing a mountain can be inferred via the universal transitivity rule. There is at least one x_13 for which x_13 will write a letter can be inferred via the existential simplification rule. There is at least one x_13 for which x_13 will listen to a song can be inferred via the existential simplification rule.