G,H,K:Graph. G 
H 
H K G Kf:(A
B), g:(BC). Bij(A; B; f) Bij(B; C; g) Bij(A; C; g o f)h:(A'A1
A2), g1:(A1B), g2:(A2C), f1:(BB'), f2:(CC'). (f1,f2) o (g1,g2) o h = (f1 o g1,f2 o g2) o hf:(AB), g:(BC), h:(CD). h o (g o f) = (h o g) o f 
AD