which is sequenced before the <code>print</code>.
</p>

<p class="rule">
For any call to <code>l.RLock</code> on a <code>sync.RWMutex</code> variable <code>l</code>,
there is an <i>n</i> such that the <i>n</i>th call to <code>l.Unlock</code>
is synchronized before the return from <code>l.RLock</code>,
and the matching call to <code>l.RUnlock</code> is synchronized before the return from call <i>n</i>+1 to <code>l.Lock</code>.

pkg sync, method (*Cond) Wait()
pkg sync, method (*Mutex) Lock()
pkg sync, method (*Mutex) Unlock()
pkg sync, method (*Once) Do(func())
pkg sync, method (*RWMutex) Lock()
pkg sync, method (*RWMutex) RLock()
pkg sync, method (*RWMutex) RLocker() Locker
pkg sync, method (*RWMutex) RUnlock()
pkg sync, method (*RWMutex) Unlock()
pkg sync, method (*WaitGroup) Add(int)
pkg sync, method (*WaitGroup) Done()