code: def find_newline(source):
      
      assert not isinstance(source, unicode)
  
      counter = collections.defaultdict(int)
      for line in source:
          if line.endswith(CRLF):
              counter[CRLF] += 1
          elif line.endswith(CR):
              counter[CR] += 1
          elif line.endswith(LF):
              counter[LF] += 1
  
      return (sorted(counter, key=counter.get, reverse=True) or [LF])[0]
docs: Return type of newline used in source.
  
      Input is a list of lines.