theorem Th3:
  a <= b & c in ['a,b'] & d in ['a,b'] & ['a,b'] c= X
  implies ['min(c,d),max(c,d)'] c= X
  proof
    assume a <= b & c in ['a,b'] & d in ['a,b'];
    then ['min(c,d),max(c,d)'] c= ['a,b'] by Lm2;
    hence thesis;
  end;
