theorem Th27:
  x in Tarski-Class X & y in Tarski-Class X implies [x,y] in Tarski-Class X
proof
  assume x in Tarski-Class X & y in Tarski-Class X;
  then {x,y} in Tarski-Class X & {x} in Tarski-Class X by Th26;
  hence thesis by Th26;
end;
