TS SetVector

I have a problem.
I change the keyboard interrupt routine to examine a register, changed the
routine back and then called the old interrupt routine inside the new
routine. Can it be done or not?
Example:
MODULE M;
  IM****T BiosIO,Lib,SYSTEM;
  VAR
     vO9:ADDRESS;
     vR:SYSTEM.Registers;
  PROCEDURE fN9():BOOLEAN;
    BEGIN
      (*Some code*)
      Lib.SetVector(9,vO9);
(*THE PROGRAM WORKS UNTIL HERE, BUT THEN I WANT TO EXECUTE THE OLD
INTERRUPT
9 ROUTINE BUT IT DOESN'T WANT TO. WHY?*)
      Lib.Intr(vR,9);
      RETURN TRUE;
  END fN9;
  BEGIN
     vO9:=Lib.GetVector(9);
     Lib.SetVector(9,ADR(fN9()));
     REPEAT
     UNTIL BiosIO.KeyPressed();
END M;