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 z_4, z_4 is poor if and only if z_4 is sad. There is at least one z_4 for which z_4 is sad or z_4 is not poor.
There is at least one z_4 for which if z_4 were poor, then z_4 is sad can be inferred via the existential rewriting rule. For all z_4, if z_4 were poor, then z_4 is sad can be inferred via the universal biconditional elimination rule. For all z_4, if z_4 were sad, then z_4 is poor can be inferred via the universal biconditional elimination rule.