Lockable

The Lockable concept extends the BasicLockable concept to include attempted locking.

For type L to be Lockable, it must meet the above condition as well as the following:

Expression	Effects	Return value
`m.try_lock()`	Attempts to acquire the lock for the current execution agent without blocking. If an exception is thrown, no lock is obtained.	`true` if the lock was acquired, `false` otherwise