class Hamster::Partitioner

@private
At the same time, it guarantees the block will only be called ONCE for each item
for which the block returns true, and another for false
This class can divide a collection into 2 ‘List`s, one of items

def done? 

def done?
  @list.empty?
end

def initialize(list, block) 

def initialize(list, block)
  @list, @block, @left, @right = list, block, [], []
end

def next_item 

def next_item
  unless @list.empty?
    item = @list.head
    (@block.call(item) ? @left : @right) << item
    @list = @list.tail
  end
end