[boost] [thread] locks.hpp double declaration of unique_lock(unique_lock&& other)

Hi in lines 248-255 of the file boost/thread/locks.hpp we found the unique_lock(unique_lock&& other) declaration twice one withoth explicit and defined and the other with explicit an undefined. #ifdef BOOST_HAS_RVALUE_REFS unique_lock(unique_lock&& other): m(other.m),is_locked(other.is_locked) { other.is_locked=false; other.m=0; } explicit unique_lock(upgrade_lock<Mutex>&& other); Could you say me why this is nesded? _____________________ Vicente Juan Botet Escribá