for G1, G2, G3 being _Graph
for F1 being PGraphMapping of G1,G2
for F2 being PGraphMapping of G2,G3 st F1 is total & rng (F1 _V) c= dom (F2 _V) & rng (F1 _E) c= dom (F2 _E) holds
F2 * F1 is total by RELAT_1:27;