code: def _tri_from_edge(self, edge):
          
          edge = tuple(edge)
          p1 = self._edges_lookup.get(edge, None)
          p2 = self._edges_lookup.get(edge[::-1], None)
          if p1 is None:
              if p2 is None:
                  raise RuntimeError("No tris connected to edge %r" % (edge,))
              return edge + (p2,)
          elif p2 is None:
              return edge + (p1,)
          else:
              raise RuntimeError("Two triangles connected to edge %r" % (edge,))
docs: Return the only tri that contains *edge*. If two tris share this
          edge, raise an exception.