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

theorem Th2:
  for x,y st x in X & y in X holds [x,y] in Total X
proof
  let x,y;
  assume x in X & y in X;
  then [x,y] in [:X,X:] by ZFMISC_1:87;
  hence thesis by EQREL_1:def 1;
end;
