code: def ranges_intersect(rset):
      
      if not rset:
          return None
  
      a = rset[0]
      for b in rset[1:]:
          if not a:
              return None
          a = range_intersect(a, b)
  
      return a
docs: Recursively calls the range_intersect() - pairwise version.
  
      >>> ranges_intersect([(48, 65), (45, 55), (50, 56)])
      [50, 55]