Oracle plsql集合中的扩展方法

这是节目

DECLARE
     TYPE t1 IS
          TABLE OF NUMBER(10);
     v_t   t1 := t1(10,20,30,40);
BEGIN
     dbms_output.put_line(v_t.first);
     dbms_output.put_line(v_t.last);
     dbms_output.put_line(v_t.PRIOR(2) );
     dbms_output.put_line(v_t.next(2) );
     v_t.extend;
     v_t.extend(2);
     v_t.extend(3,2);
     v_t(5) := 50;
     v_t(6) := 60;
     v_t(7) := 70;
     v_t.trim;
     dbms_output.put_line(v_t.count);
     FOR i IN v_t.first..v_t.last LOOP
          dbms_output.put_line(v_t(i) );
     END LOOP;
END;
/

输出：

这里我无法理解这个程序的输出，请任何人解释一下这个输出。 提前谢谢

1 = dbms_output.put_line(v_t.first) - returns the first subscript/index value of 
    t1(10,20,30,40) which is 1
4 = dbms_output.put_line(v_t.last) - returns the last subscript/index value of 
    t1(10,20,30,40) which is 4
1 = dbms_output.put_line(v_t.PRIOR(2)) - returns the subscript that precedes index n in a 
    collection which is 1. 
3 = dbms_output.put_line(v_t.next(2)) - returns the subscript that succeeds index n in a 
    collection which is 3.
9 = dbms_output.put_line(v_t.count) - v_t.EXTEND appends one null element to the collection, v_t.EXTEND appends 2 null elements to the collection and v_t.EXTEND(3,2) appends 3 copies of the 2nd element to the collection
10 = dbms_output.put_line(v_t(i) - Prints all the elements.
20
30
40
50
60
70
20 = v_t.extend(3,2) - Added 3 copies of 2nd element which is 20
20 = v_t.extend(3,2) -                   ""

---

第三份副本（20）被排除在外，因为v_t.trim从集合末尾删除了一个元素

您没有明确理解哪一部分？请仔细阅读文档以了解收集方法：感谢您宝贵的解释

1 = dbms_output.put_line(v_t.first) - returns the first subscript/index value of 
    t1(10,20,30,40) which is 1
4 = dbms_output.put_line(v_t.last) - returns the last subscript/index value of 
    t1(10,20,30,40) which is 4
1 = dbms_output.put_line(v_t.PRIOR(2)) - returns the subscript that precedes index n in a 
    collection which is 1. 
3 = dbms_output.put_line(v_t.next(2)) - returns the subscript that succeeds index n in a 
    collection which is 3.
9 = dbms_output.put_line(v_t.count) - v_t.EXTEND appends one null element to the collection, v_t.EXTEND appends 2 null elements to the collection and v_t.EXTEND(3,2) appends 3 copies of the 2nd element to the collection
10 = dbms_output.put_line(v_t(i) - Prints all the elements.
20
30
40
50
60
70
20 = v_t.extend(3,2) - Added 3 copies of 2nd element which is 20
20 = v_t.extend(3,2) -                   ""