@pimms.value<EOL><INDENT>def edge_face_index(edge_data):<DEDENT>
return edge_data['<STR_LIT>']<EOL>
tess.edge_face_index is a mapping that indexes the edges by vertex labels (not vertex
  indices) to a face index or pair of face indices. So for an edge from the vertex labeled
  u to the vertex labeled v, index.edge_face_index[(u,v)] is a tuple of the faces that are
  adjacent to the edge (u,v).