code: def _edge_below_front(self, edge, front_index):
          
          f0 = self._front[front_index-1]
          f1 = self._front[front_index+1]
          return (self._orientation(edge, f0) > 0 and 
                  self._orientation(edge, f1) < 0)
docs: Return True if *edge* is below the current front. 
          
          One of the points in *edge* must be _on_ the front, at *front_index*.