class Hamster::Splitter

@private
item
after that. It guarantees that the block will only be called ONCE for each
elements for which the block returns true, and another for all the elements
This class can divide a list up into 2 ‘List`s, one for the prefix of

def done? 

def done?
  @list.empty?
end

def initialize(list, block) 

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

def next_item 

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