What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? p_20 -> t_4. t_4 -> p_20. u_4 or p_10. p_10 -> p_20. u_4 -> t_4. u -> ~t_4.
p_10 -> t_4. u_4 -> p_20. p_20 <-> t_4. t_4 <-> p_20.