code: def _adjacent_tri(self, edge, i):
          
          if not np.isscalar(i):
              i = [x for x in i if x not in edge][0]
  
          try:
              pt1 = self._edges_lookup[edge]
              pt2 = self._edges_lookup[(edge[1], edge[0])]
          except KeyError:
              return None
              
          if pt1 == i:
              return (edge[1], edge[0], pt2)
          elif pt2 == i:
              return (edge[1], edge[0], pt1)
          else:
              raise RuntimeError("Edge %s and point %d do not form a triangle "
                                 "in this mesh." % (edge, i))
docs: Given a triangle formed by edge and i, return the triangle that shares
          edge. *i* may be either a point or the entire triangle.