for C being Cocartesian_category
for a, b, c, d, e, s being Object of C
for f being Morphism of a,c
for h being Morphism of b,d
for g being Morphism of c,e
for k being Morphism of d,s st Hom (a,c) <> {} & Hom (b,d) <> {} & Hom (c,e) <> {} & Hom (d,s) <> {} holds
(g + k) * (f + h) = (g * f) + (k * h)