code: def size(filename, use_cache_only=False):
      
      filename = unmap_file(filename)
      if filename not in file_cache:
          if not use_cache_only: cache_file(filename)
          if filename not in file_cache:
              return None
          pass
      return len(file_cache[filename].lines['plain'])
docs: Return the number of lines in filename. If `use_cache_only' is False,
      we'll try to fetch the file if it is not cached.