def add_comment(options={})
      raise ArgumentError, "Comment require an author" unless options[:author]
      raise ArgumentError, "Comment requires text" unless options[:text]
      raise ArgumentError, "Comment requires ref" unless options[:ref]
      self << Comment.new(self, options)
      yield last if block_given?
      last
    end
Adds a new comment to the worksheet that owns these comments.
@note the author, text and ref options are required
@option options [String] author The name of the author for this comment
@option options [String] text The text for this comment
@option options [Stirng|Cell] ref The cell that this comment is attached to.