docs: def highest(self) -> CtsSinglePassageId:
          """ Return highest reference level
  
          For references such as 1.1-1.2.8, with different level, it can be useful to access to the highest node in the
          hierarchy. In this case, the highest level would be 1.1. The function would return ["1", "1"]
  
          .. note:: By default, this property returns the start level
  
          :rtype: CtsReference
          """
          if not self.end:
              return self.start
          elif len(self.start) < len(self.end) and len(self.start):
              return self.start
          elif len(self.start) > len(self.end) and len(self.end):
              return self.end
          elif len(self.start):
              return self.start