code: def existing_subpath(root_path, valid_subpaths, tiebreaker='first',
                       verbose=VERYVERBOSE):
      
      # Find the oxford_style groundtruth directory
      for subpath in valid_subpaths:
          path  = join(root_path, subpath)
          if checkpath(path, verbose=verbose):
              if tiebreaker == 'first':
                  return path
      raise AssertionError('none of the following subpaths exist: %r' %
                           (valid_subpaths,))
docs: Returns join(root_path, subpath) where subpath in valid_subpath ane
      exists(subpath)