theorem
  z in [:{x1,x2},Y:] implies (z`1=x1 or z`1=x2) & z`2 in Y
proof
  assume
A1: z in [:{x1,x2},Y:];
  then z`1 in {x1,x2} by Th4;
  hence thesis by A1,Th4,TARSKI:def 2;
end;
