The results obtained in Section 3 and 4 also hold for any lextensive categories . Denote by 0 an initial and 1 a terminal object. The dual notion is a strict terminal object . The initial 0 is strict. But 0 is strict.
S be two maps.
T 2 be four maps. X 2 be fours maps. Finite sums commute with equalizers. X 2 be two maps. One direction is obvious because any sum of epis is epic.