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: If David were a bartender, then Susan is a sailor. For all x_13, x_13 is a sailor if and only if x_13 is a scientist. David is a bartender.
Susan is a sailor can be inferred via the modus ponens rule. For all x_13, if x_13 were a sailor, then x_13 is a scientist can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 were a scientist, then x_13 is a sailor can be inferred via the universal biconditional elimination rule.