drop usage for list;

[xueyue]

here is a minor problem I met:
suppose I have a object, x, and:
[x displayList: list]; 
will display the content of 'list'. If list=nil, or 
list has no content, i.e. [list getCount]==0, then 
-displayList will simply do nothing, otherwise, the 
members of the list will be displayed.
The following will work:
(1) list=[List create: [self getZone]];
    [x displayList: list];
     [list drop];
     list=nil;
     [x displayList: list];
     ....
however, the following will not work:
(2) list=[List create: [self getZone]];
    [list drop];
     [x displayList: list];
    ......

The problem seems come from that after [list drop] is 
executed, list does not exist anymore, and it is not 
'nil' either. Is it NULL?

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