theorem
  for G being _Graph, X, Y being set
  holds G.edgesDBetween(X,Y) c= G.edgesBetween(X,Y)
proof
  let G be _Graph, X, Y be set;
  now
    let e be object;
    assume e in G.edgesDBetween(X,Y);
    then e DSJoins X,Y,G by Def31;
    then e SJoins X,Y,G;
    hence e in G.edgesBetween(X,Y) by Def30;
  end;
  hence thesis;
end;
