code: public Instruction add(Instruction _instruction) {
  
        if (head == null) {
           head = _instruction;
        } else {
           _instruction.setPrevExpr(tail);
           tail.setNextExpr(_instruction);
  
        }
  
        tail = _instruction;
        logger.log(Level.FINE, "After PUSH of " + _instruction + " tail=" + tail);
        return (tail);
     }
docs: Add this instruction to the end of the list.
  
  @param _instruction
  @return The instruction we added