code: def add_arrow(self, tipLoc, tail=None, arrow=arrow.default):
          
          self._arrows.append((tipLoc, tail, arrow))
docs: This method adds a straight arrow that points to
          @var{TIPLOC}, which is a tuple of integers. @var{TAIL}
          specifies the starting point of the arrow. It is either None
          or a string consisting of the following letters: 'l', 'c',
          'r', 't', 'm,', and 'b'.  Letters 'l', 'c', or 'r' means to
          start the arrow from the left, center, or right of the text
          box, respectively. Letters 't', 'm', or 'b' means to start the
          arrow from the top, middle or bottom of the text box.  For
          example, when @samp{tail = 'tc'} then arrow is drawn from
          top-center point of the text box. ARROW specifies the style of
          the arrow. <<arrow>>.