reserve W,X,Y,Z for set,
  f,g for Function,
  a,x,y,z for set;
reserve u,v for Element of Tarski-Class(X),
  A,B,C for Ordinal,
  L for Sequence;

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;
