   RELEASE_LOCK(str)
Releases the lock named by the string str that was obtained with
GET_LOCK(). Returns 1 if the lock was released, 0 if the
lock wasn't locked by this thread (in which case the lock is not released),
and NULL if the named lock didn't exist.  The lock will not exist if
it was never obtained by a call to GET_LOCK() or if it already has
been released.

The DO statement is convenient to use with RELEASE_LOCK().
See also : [DO,  , DO].
