code: def tags(norm):
      '''
      Divide a normalized tag string into hierarchical layers.
      '''
      # this is ugly for speed....
      parts = norm.split('.')
      return ['.'.join(parts[:i]) for i in range(1, len(parts) + 1)f tags(norm):
      '''
      Divide a normalized tag string into hierarchical layers.
      '''
      # this is ugly for speed....
      parts = norm.split('.')
      return ['.'.join(parts[:i]) for i in range(1, len(parts) + 1)]
docs: Divide a normalized tag string into hierarchical layers.