theorem Th37:
  for G being _Graph, X1,X2,Y1,Y2 being set st X1 c= X2 & Y1 c= Y2 holds
  G.edgesBetween(X1,Y1) c= G.edgesBetween(X2,Y2)
proof
  let G be _Graph, X1,X2,Y1,Y2 be set;
  assume
A1: X1 c= X2 & Y1 c= Y2;
    let e be object;
    assume e in G.edgesBetween(X1,Y1);
    then e SJoins X1,Y1,G by Def30;
    then e SJoins X2,Y2,G by A1;
    hence e in G.edgesBetween(X2,Y2) by Def30;
end;
