code: def overlap_size(self,tx2):
      
      total = 0
      for e1 in self.exons:
        for e2 in tx2.exons:
          total += e1.overlap_size(e2)
      return total
docs: Return the number of overlapping base pairs between two transcripts
  
      :param tx2: Another transcript
      :type tx2: Transcript
      :return: overlap size in base pairs
      :rtype: int