docs: def _find_common_roots(paths):
      """Out of some paths it finds the common roots that need monitoring."""
      paths = [x.split(os.path.sep) for x in paths]
      root = {}
      for chunks in sorted(paths, key=len, reverse=True):
          node = root
          for chunk in chunks:
              node = node.setdefault(chunk, {})
          node.clear()
  
      rv = set()
  
      def _walk(node, path):
          for prefix, child in iteritems(node):
              _walk(child, path + (prefix,))
          if not node:
              rv.add("/".join(path))
  
      _walk(root, ())
      return rv