docs: def set_of_vars(lovs):
      """Build set of variables from list.
  
      Args:
          lovs: nested lists of variables such as the one produced by
              :func:`list_of_vars`.
      Returns:
          set of str: flattened set of all the variables present in the
          nested lists.
      """
      return set(var for pvars in lovs for svars in pvars for var in svars)