theorem
  z in [:{x1,x2},{y}:] implies (z`1=x1 or z`1=x2) & z`2 = y
proof
  assume z in [:{x1,x2},{y}:];
  then z`1 in {x1,x2} & z`2 in {y} by Th4;
  hence thesis by TARSKI:def 1,def 2;
end;
