formula_list(usable).

(	all x ( n(x,I)=n(r(x),I) )).

(	all x y ( n(k(x,n(r(x),y)),I) = n(k(x,y),I) )).

(	all x y ( n(k(n(x,r(y)),n(r(x),y)),I) = n(k(x,y),I) )).

end_of_list.