theorem Th9: :: NATTRA_1:13

for A, B being Category
for F being Functor of A,B
for a, b, c being Object of A st Hom (a,b) <> {} & Hom (b,c) <> {} holds
for f being Morphism of a,b
for g being Morphism of b,c holds F /. (g * f) = (F /. g) * (F /. f)