inno db 隔离级别和锁定

I am reading a manual about innodb transactions but still, there is lots of unclear stuff to me. For instance, I don't quite understand to the following behaviour:

-- client 1                             -- client 2
mysql> create table simple (col int) 
       engine=innodb; 

mysql> insert into simple values(1);
Query OK, 1 row affected (0.00 sec)

mysql> insert into simple values(2);
Query OK, 1 row affected (0.00 sec)

mysql> select @@tx_isolation;                                                              
+-----------------+                                                                         
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |                                                                         
+-----------------+

mysql> begin;                                    
Query OK, 0 rows affected (0.01 sec)            
                                        mysql> begin;
                                        Query OK, 0 rows affected (0.00 sec)

mysql> update simple set col=10 where col=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

                                         mysql> update simple set col=42 where col=2;
                                         -- blocks

现在，最后一个更新命令(在客户端 2 中)等待.我希望该命令能够执行，因为我认为只有第 1 行被锁定.即使客户端 2 中的第二个命令是 insert，行为也是相同的.谁能描述一下这个例子背后的锁定背景(锁定的位置和原因)?