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_4, if x_4 were a doctor, then x_4 is a mechanic. For all x_4, if x_4 plays a game, then x_4 is a doctor. There is at least one x_4 for which x_4 will play a game. For all x_4, if x_4 takes a plane, then x_4 is a mechanic.
There is at least one x_4 for which x_4 is a doctor can be inferred via the existential modus ponens rule. For all x_4, if x_4 plays a game, then x_4 is a mechanic can be inferred via the universal transitivity rule.