No method error for "acts_as_tree" ???

I'm running Rails 2.0 and I'm getting this error (undefined method
`acts_as_tree') when trying to use the 'acts_as_tree' functionality in
one of my models:

class Topic < ActiveRecord::Base
  has_many :pages
  has_many :screens, :through => :pages
  has_many :folders
  has_many :courses, :through => :folders
  acts_as_tree :order => "title"
end

I have the "topic" table in my db setup as follows:

create_table :topics do |t|
  t.string :title
  t.integer :parent_id
end

What gives?

Did this get replaced by a plugin?

I've found a similar construct called "acts_as_ordered_tree" that I
could use as a plugin, but I want to be sure before going forward.

Thanks,

Yes, acts_as_tree (acts_as_list, acts_as_nested_set, autocomplete,...)
were removed from Rails core and released separately as plugins in
2.x.