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 x_11, x_11 is sad if and only if x_11 is rich. There is at least one x_11 for which if x_11 were sad, then x_11 is rich. For all x_11, if x_11 writes a letter, then x_11 is rich.
For all x_11, if x_11 were sad, then x_11 is rich can be inferred via the universal biconditional elimination rule. For all x_11, if x_11 were rich, then x_11 is sad can be inferred via the universal biconditional elimination rule.