sign3 (#420)

     When you first went west from the cave entrance, you had to be carrying a miner's 
hat before you could exit to the west.  That exit was LOCKED, and the miner's hats 
were KEYS.  In that case, the exit "west" was locked as follows:
     
             @lock west with blue hat || red hat || green hat
     
     This means the exit is passable to anyone carrying the blue hat, OR the red hat OR 
the green hat.  Players are prevented from carrying the hats out to the river bank 
with the following lock on the eastward exit:
     
             @lock east with !red hat && ! green hat
     
     So the exit is open if you are not carrying the red hat AND not carrying the green 
hat, etc.  Other kinds of objects can be @locked.  For example, the signs in this 
cave cannot be picked up or moved by anyone but Calkins, because he typed:
     
             @lock sign with me