theorem Th13:
  <*x*> ==>. y/"(x\y) & <*x*> ==>. (y/"x)\y
proof
A1: <*y/"x*>^<*x*> ==>. y by Th12;
  <*x*>^<*x\y*> ==>. y by Th12;
  hence thesis by A1,Def18;
end;
