def boundary_vertices(self, distance, outer=False, indices=False):
<EOL>raise NotImplementedError('<STR_LIT>')<EOL>
path.boundary_vertices(d) yields an array of vertex labels representing all vertices that
  are both on the on the inner/left-hand side of the given path and within a distance d of
  the boundary of the path.

The following options may be given:
  * outer (default: False) may be set to True in order to use the right-hand/outer side of
    the path instead of the inner/left-hand side.
  * indices (default: False) may be set to True in order to yield indices instead of labels.