code: def _get_index(self):
          
          glyph = self.glyph
          if glyph is not None:
              parent = glyph
          else:
              parent = self.font
          if parent is None:
              return None
          return parent.guidelines.index(self)
docs: Get the guideline's index.
          This must return an ``int``.
  
          Subclasses may override this method.