reserve X,Y,Z,x,y,z for set;

theorem
  for x,y st x in field Total X & y in field Total X holds [x,y] in Total X
proof
  let x,y;
  assume x in field Total X & y in field Total X;
  then x in X & y in X by ORDERS_1:12;
  hence thesis by Th2;
end;
