# File lib/neovim/buffer.rb, line 104
    def append(index, str)
      window = @session.request(:vim_get_current_window)
      cursor = window.cursor

      if index < 0
        raise ArgumentError, "Index out of bounds"
      else
        set_lines(index, index, true, [str])
        window.set_cursor(cursor)
      end
      str
    end