title: Boost UpgradeLockable Concept
body: <p>Why Boost UpgradeLockable Concept (<a href="http://www.boost.org/doc/libs/1_57_0/doc/html/thread/synchronization.html#thread.synchronization.mutex_concepts.upgrade_lockable" rel="nofollow">http://www.boost.org/doc/libs/1_57_0/doc/html/thread/synchronization.html#thread.synchronization.mutex_concepts.upgrade_lockable</a>) have unlock_and_lock_* and unlock_upgrade_and_lock_* but don't have unlock_shared_and_lock_<em>? It have try_unlock_shared_and_lock_</em> but only when BOOST_THREAD_PROVIDES_SHARED_MUTEX_UPWARDS_CONVERSIONS is available and I don't want to "try". There's some restriction about doing such operations?</p>