class Rectangle
  include Model

  attr_accessor :left, :right, :top, :bottom, :width

  def computed_width
    return unless left and right
    right - left
  end

  def effective_width
    width || computed_width
  end

  # ... etc ...

  def effective
    Rectangle.new(
      left: effective_left,
      right: effective_right,
      width: effective_width,
      # ... etc
    )
  end
end