如何在PL/SQL中实现堆栈？

CREATE OR REPLACE PROCEDURE init_stack(stack OUT SYS.ODCIVARCHAR2LIST) IS
BEGIN
  stack := SYS.ODCIVARCHAR2LIST();
END;
/

CREATE OR REPLACE PROCEDURE push(stack IN OUT SYS.ODCIVARCHAR2LIST, element IN VARCHAR2) IS
BEGIN
  stack.EXTEND;
  stack(stack.COUNT) := element;
END;
/

CREATE OR REPLACE PROCEDURE pop(stack IN OUT SYS.ODCIVARCHAR2LIST, element OUT VARCHAR2) IS
BEGIN
  IF stack.COUNT > 0 THEN
    element := stack(stack.COUNT);
    stack.DELETE(stack.COUNT);
  ELSE
    element := NULL;
  END IF;
END;
/

CREATE OR REPLACE FUNCTION is_empty(stack IN SYS.ODCIVARCHAR2LIST) RETURN BOOLEAN IS
BEGIN
  RETURN stack.COUNT = 0;
END;
/

DECLARE
  stack SYS.ODCIVARCHAR2LIST;
  element VARCHAR2(100);
BEGIN
  init_stack(stack);
  
  push(stack, 'Element 1');
  push(stack, 'Element 2');
  
  IF NOT is_empty(stack) THEN
    pop(stack, element);
    DBMS_OUTPUT.PUT_LINE('Popped element: ' || element);
  END IF;
  
  IF NOT is_empty(stack) THEN
    pop(stack, element);
    DBMS_OUTPUT.PUT_LINE('Popped element: ' || element);
  END IF;
  
  IF is_empty(stack) THEN
    DBMS_OUTPUT.PUT_LINE('Stack is empty');
  END IF;
END;
/