code: def get_line_in_facet(self, facet):
          
  
          lines = list(facet.outer_lines)
          pt = []
          prev = None
          while len(lines) > 0:
              if prev is None:
                  l = lines.pop(0)
              else:
                  for i, l in enumerate(lines):
                      if prev in l:
                          l = lines.pop(i)
                          if l[1] == prev:
                              l.reverse()
                          break
              # make sure the lines are connected one by one.
              # find the way covering all pts and facets
              pt.append(self.wulff_pt_list[l[0]].tolist())
              pt.append(self.wulff_pt_list[l[1]].tolist())
              prev = l[1]
  
          return pt
docs: Returns the sorted pts in a facet used to draw a line