code: def chan_path_from_rel_path(rel_path, channeldir):
      
      rel_path_parts = rel_path.split(os.path.sep)
      dirs_before_channeldir = channeldir.split(os.path.sep)[:-1]
      channel_chan_path = []  # path relative to channel root, inclusive
      for idx, part in enumerate(rel_path_parts):
          if idx < len(dirs_before_channeldir) and dirs_before_channeldir[idx]==part:
              continue
          else:
              channel_chan_path.append(part)
      chan_path = os.path.join(*channel_chan_path)
      return chan_path
docs: Convert `rel_path` form os.walk tuple format to a tuple of directories and
      subdirectories, starting with the `channeldir` folder, e.g.,::
  
          >>> chan_path_from_rel_path('content/open_stax_zip/Open Stax/Math/Elementary',
                                  'content/open_stax_zip/Open Stax')
          'Open Stax/Math/Elementary'