theorem
  z in [:{x},{y1,y2}:] implies z`1 = x & (z`2 = y1 or z`2 = y2)
proof
  assume z in [:{x},{y1,y2}:];
  then z`1 in {x} & z`2 in {y1,y2} by Th4;
  hence thesis by TARSKI:def 1,def 2;
end;
