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: There is at least one x_10 for which x_10 is a writer if and only if x_10 is an artist. For all x_10, if x_10 were sad, then x_10 is a janitor. For all x_10, x_10 is not a janitor. For all x_10, x_10 is sad or x_10 is a writer.
For all x_10, x_10 is not sad can be inferred via the universal modus tollens rule. There is at least one x_10 for which if x_10 were a writer, then x_10 is an artist can be inferred via the existential biconditional elimination rule. There is at least one x_10 for which if x_10 were an artist, then x_10 is a writer can be inferred via the existential biconditional elimination rule.