def valid?
      return false if @first_page.nil?
      return false if @last_page.nil?
      return false if @odd_pages.nil?
      return false if @even_pages.nil?
      return true
    end
def valid?
      return false if @first_page.nil?
      return false if @last_page.nil?
      return false if @odd_pages.nil?
      return false if @even_pages.nil?
      return true
    end
Check to see if the all the properties in the model are valid
 @return true if the model is valid