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_21 for which x_21 is a bartender. For all x_21, x_21 will read a book if and only if x_21 is a writer. For all x_21, if x_21 reads a book, then x_21 is a taxi driver. For all x_21, if x_21 were a bartender, then x_21 is a janitor. For all x_21, if x_21 were a janitor, then x_21 is a writer.
There is at least one x_21 for which x_21 is a janitor can be inferred via the existential modus ponens rule. For all x_21, if x_21 were a bartender, then x_21 is a writer can be inferred via the universal transitivity rule. For all x_21, if x_21 reads a book, then x_21 is a writer can be inferred via the universal biconditional elimination rule. For all x_21, if x_21 were a writer, then x_21 is reading a book can be inferred via the universal biconditional elimination rule.