for G being _finite _Graph
for V being VertexScheme of G holds
( V is perfect iff for a, b, c being Vertex of G st b <> c & a,b are_adjacent & a,c are_adjacent holds
for va, vb, vc being Nat st va in dom V & vb in dom V & vc in dom V & V . va = a & V . vb = b & V . vc = c & va < vb & va < vc holds
b,c are_adjacent )