```ruby
#
# Copyright (c) 2013 by Lifted Studios.  All Rights Reserved.
#

module KangaRuby
  # Creates a diagram and writes it out to an `SVG` file.  A diagram is represented by a collection of objects.
  class Diagram
    # Set of arrows in the diagram, from top to bottom.
    attr_reader :arrows

    # Set of lifelines in the diagram, from left to right.
    attr_reader :lifelines

    # Creates an empty diagram.
    def initialize
      @arrows = []
      @lifelines = []
    end
  end
end
```

Add some example code to Diagram documentation
user
```ruby
#
# Copyright (c) 2013 by Lifted Studios.  All Rights Reserved.
#

module KangaRuby
  # Creates a diagram by constructing an `SVG` document and returning it as a string.
  #
  # @example Basic flow
  #   diagram = Diagram.new
  #
  #   diagram.lifelines << Lifeline.new('Alice')
  #   diagram.lifelines << Lifeline.new('Bob')
  #   diagram.arrows << Arrow.new(1, 2)
  #
  #   diagram.draw
  class Diagram
    # Set of arrows in the diagram, from top to bottom.
    attr_reader :arrows

    # Set of lifelines in the diagram, from left to right.
    attr_reader :lifelines

    # Creates an empty diagram.
    def initialize
      @arrows = []
      @lifelines = []
    end
  end
end
```
assistant