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 plays squash, then x_13 is playing tennis. For all x_13, if x_13 drinks water, then x_13 is playing squash. For all x_13, x_13 will play squash if and only if x_13 will play tennis. There is at least one x_13 for which x_13 will write a letter. For all x_13, x_13 will drink water or x_13 will play tennis.
For all x_13, if x_13 drinks water, then x_13 is playing tennis can be inferred via the universal transitivity rule. For all x_13, if x_13 plays squash, then x_13 is playing tennis can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 plays tennis, then x_13 is playing squash can be inferred via the universal biconditional elimination rule.