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 writes a letter, then x_13 is an astronaut. For all x_13, x_13 is not an astronaut or x_13 will work. For all x_13, if x_13 were a poet, then x_13 is an astronaut. There is at least one x_13 for which x_13 will write a letter.
There is at least one x_13 for which x_13 is an astronaut can be inferred via the existential modus ponens rule.