@open <exit> [=<object> [; <object2>; ... <objectn> ] [=<regname>]]. Opens an exit in the current room, optionally attempting to link it simultaneously. If a <regname> is specified, then the _reg/<regname> property on the player is set to the dbref of the new object. This lets players refer to the object as $<regname> (ie: $mybutton) in @locks, @sets, etc. Opening an exit costs a penny, and an extra penny to link it, and you must control the room where it is being opened.