theorem
  for G2, G1 holds
    the_Source_of G2 = (the_Source_of G1) | the_Edges_of G2 &
    the_Target_of G2 = (the_Target_of G1) | the_Edges_of G2
proof
  let G2, G1;
  G2 is Subgraph of G1 by Th61;
  hence thesis by GLIB_000:45;
end;
