why would enable constraint cause a delete to wait on library cache lock?

I am only an egg, so I hope someone else can explain this to me.

Oracle Enterprise Edition 8.1.7.4.1 on Windows 2000

I was experimenting to see if an "alter table enable validate constraint" would cause DML statements to wait. I thought it wouldn't. But in real life I see something different.

I have a table X (object_id 429995) with about 100 million rows. In one session (sid 15) I enable a check constraint, and in another session (sid 14) I simultaneously delete a few rows from the table. The delete "waits" on the enable constraint to complete, and it's waiting on a library cache lock. Why would that be? At the end of this e-mail you can see the contents of DBA_LOCKS, V$LOCKED_OBJECT, V$SESSION_WAIT. I read the system state dump and I still don't understand why there would be a library cache lock.

Session ID 15
--- sid-15-SQL1

alter table hes_a_keeper.many_rows add
  (constraint ck1 check (dummy_column > 'B') disable) ;
--- sid-15-SQL2 (simultaneous with sid-14-SQL1)
alter table hes_a_keeper.many_rows enable validate constraint ck1 ;

Session ID 14
--- sid-14-SQL1 (simultaneous with sid-15-SQL2)
delete from hes_a_keeper.many_rows where rownum < 10 ;

Session ID 10
--- statements issued while sid-14-SQL1 and sid-15-SQL2 are running
alter session set events 'immediate trace name systemstate level 10' ;

select * from dba_locks where session_id in (14,15) ;
select * from v$locked_object where object_id = 429995 ;
select * from v$session_wait where sid in (14,15) ;


SQL> select * from dba_locks where session_id in (14,15) ;
SESSION_ID LOCK_TYPE                  MODE_HELD
---------- -------------------------- ----------------------------------------
MODE_REQUESTED                           LOCK_ID1
---------------------------------------- ----------------------------------------
LOCK_ID2                                 LAST_CONVERT BLOCKING_OTHERS
---------------------------------------- ------------ ----------------------------------------
        15 DML                        Share
None                                     429995
0                                                  10 Not Blocking

SQL> select * from v$locked_object where object_id = 429995 ;

   XIDUSN XIDSLOT XIDSQN OBJECT_ID SESSION_ID ORACLE_USERNAME

--------- --------- --------- --------- ---------- ------------------------------
OS_USER_NAME                   PROCESS   LOCKED_MODE
------------------------------ --------- -----------
        0         0         0    429995         15 JRK
jkilchoe                       1540:260            4


SQL> select * from v$session_wait where sid in (14,15) ;
      SID      SEQ# EVENT
--------- --------- ----------------------------------------------------------------
P1TEXT                                                                  P1 P1RAW
---------------------------------------------------------------- --------- --------
P2TEXT                                                                  P2 P2RAW
---------------------------------------------------------------- --------- --------
P3TEXT                                                                  P3 P3RAW    WAIT_TIME
---------------------------------------------------------------- --------- -------- ---------

--------------- -------------------
       15      5005 db file scattered read
file#                                                                   13 0000000D
block#                                                               19516 00004C3C
blocks                                                                   8 00000008         0
              0 WAITING

       14       142 library cache lock
handle address                                                    47483828 02D48BB4
lock address                                                      40114204 0264181C
10*mode+namespace                                                       21 00000015         0
              9 WAITING

SQL>

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-- 
Author: Jacques Kilchoer
  INET: Jacques.Kilchoer_at_quest.com

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