theorem
  X c= Y implies G.edgesBetween(X) c= G.edgesBetween(Y)
proof
  assume
A1: X c= Y;
    let x be object;
    assume
A2: x in G.edgesBetween(X);
    then (the_Source_of G).x in X & (the_Target_of G).x in X by Lm5;
    hence x in G.edgesBetween(Y) by A1,A2,Lm5;
end;
