theorem
  z in [:X,{y1,y2}:] implies z`1 in X & (z`2 = y1 or z`2 = y2)
proof
  assume
A1: z in [:X,{y1,y2}:];
  then z`2 in {y1,y2} by Th4;
  hence thesis by A1,Th4,TARSKI:def 2;
end;
