The @lock command is used to control which players
can get a particular object or pass through a particular 
exit. Locks can refer to both players and objects. If
a lock refers to a player, then that player can use
the object. If a lock refers to an object, then the
player can use it if the player is carrying the
specified key. Objects and players can be combined
using & (AND), | (OR), ! (NOT), and ( and )
(parentheses to group expressions).

If players are mentioned in the lock, it is crucial
that an * appear before the name of each player.

To clear a lock, place nothing on the right-hand side
of the = sign.


@lock north = *jessica
@lock south = (!*roger) & blue key