theorem
  <*>the carrier of s ==>. (y/"(x\y))/"x &
  <*>the carrier of s ==>. x\((y/"x)\ y )
proof
A1: <*x*> ==>. y/"(x\y) by Th13;
  <*x*> ==>. (y/"x)\y by Th13;
  hence thesis by A1,Th19;
end;
