theorem Th7:
 [x,y] in X implies y in union union X
  proof assume
A1: [x,y] in X;
   {x,y} in {{x},{x,y}} by TARSKI:def 2;
   then
A2: {x,y} in union X by A1,TARSKI:def 4;
    y in {x,y} by TARSKI:def 2;
   hence y in union union X by A2,TARSKI:def 4;
  end;
