theorem Th6:
 [x,y] in X implies x in union union X
  proof assume
A1: [x,y] in X;
   {x} in {{x},{x,y}} by TARSKI:def 2;
   then
A2: {x} in union X by A1,TARSKI:def 4;
    x in {x} by TARSKI:def 1;
   hence x in union union X by A2,TARSKI:def 4;
  end;
