X are two strong subobjects. Y be the pullback of x along f . The map m 1 as the pullback of the strong mono m is a strong mono. Thus the pullback e 1 of the epi e along t is also epi. Y 1 of x and f is 0 .
T is an isomorphism as it is an epic strong mono.
It follows that f and m are disjoint. Since f -1 is bounded and preserves meets, we only need to verify that it also preserves joins. X are two strong subobjects. X be the map induced by u and v. Y is a strong mono.