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: For all y, y is rich if and only if y is sad. If Linda writes a letter, then Barbara is sad. For all y, y is not rich.
For all y, if y were rich, then y is sad can be inferred via the universal biconditional elimination rule. For all y, if y were sad, then y is rich can be inferred via the universal biconditional elimination rule. For all y, y is not sad can be inferred via the universal biconditional elimination rule.