Categorical Geometry Chapter 1 Section 1.5 Zhaohua Luo 1.5.
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.