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: Elisabeth is sleeping. Joseph is not writing a letter. For all x_13, x_13 will write a letter if and only if x_13 is a poet.
Elisabeth is sleeping and Joseph is not writing a letter can be inferred via the conjunction rule. Joseph is not writing a letter and Elisabeth is sleeping can be inferred via the conjunction rule. For all x_13, if x_13 writes a letter, then x_13 is a poet can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 were a poet, then x_13 is writing a letter can be inferred via the universal biconditional elimination rule.