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_21, x_21 will work or x_21 will play squash. For all x_21, x_21 is a poet if and only if x_21 will write a letter. For all x_21, if x_21 works, then x_21 is a poet. For all x_21, if x_21 plays squash, then x_21 is a poet.
For all x_21, x_21 is a poet can be inferred via the universal disjunction elimination rule. For all x_21, if x_21 were a poet, then x_21 is writing a letter can be inferred via the universal biconditional elimination rule. For all x_21, if x_21 writes a letter, then x_21 is a poet can be inferred via the universal biconditional elimination rule.