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: Mary is a sailor if and only if David is working. Joseph is not making tea. For all x_12, x_12 will work if and only if x_12 is sad. If David were sad, then Joseph is making tea.
David is not sad can be inferred via the modus tollens rule. If Mary were a sailor, then David is working can be inferred via the biconditional elimination rule. If David works, then Mary is a sailor can be inferred via the biconditional elimination rule. For all x_12, if x_12 works, then x_12 is sad can be inferred via the universal biconditional elimination rule. For all x_12, if x_12 were sad, then x_12 is working can be inferred via the universal biconditional elimination rule.