Class 'TreeBehavior' not found

when class GroupTreeBehavior extending class TreeBehavior, I got this
error:

[quote]
Fatal error: Class 'TreeBehavior' not found in /var/www/cakephp/app/
models/behaviors/group_tree.php on line 2
[/quote]

line 2 is:

[code]
class GroupTreeBehavior extends TreeBehavior
[/code]

what's the problem ?

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