code: def faces_unique_edges(self):
          
          # make sure we have populated unique edges
          populate = self.edges_unique
          # we are relying on the fact that edges are stacked in triplets
          result = self._cache['edges_unique_inverse'].reshape((-1, 3))
          return result
docs: For each face return which indexes in mesh.unique_edges constructs
          that face.
  
          Returns
          ---------
          faces_unique_edges : (len(self.faces), 3) int
            Indexes of self.edges_unique that
            construct self.faces
  
          Examples
          ---------
          In [0]: mesh.faces[0:2]
          Out[0]:
          TrackedArray([[    1,  6946, 24224],
                        [ 6946,  1727, 24225]])
  
          In [1]: mesh.edges_unique[mesh.faces_unique_edges[0:2]]
          Out[1]:
          array([[[    1,  6946],
                  [ 6946, 24224],
                  [    1, 24224]],
                 [[ 1727,  6946],
                  [ 1727, 24225],
                  [ 6946, 24225]]])