docs: def _inv_key(list_keys, valid_keys):
      """
      -----
      Brief
      -----
      A sub-function of _filter_keywords function.
  
      -----------
      Description
      -----------
      Function used for identification when a list of keywords contains invalid keywords not present
      in the valid list.
  
      ----------
      Parameters
      ----------
      list_keys : list
          List of keywords that must be verified, i.e., all the inputs needs to be inside valid_keys
          in order to a True boolean be returned.
  
      valid_keys : list
          List of valid keywords.
  
      Returns
      -------
      out : boolean, list
          Boolean indicating if all the inserted keywords are valid. If true a list with invalid
          keywords will be returned.
      """
  
      inv_keys = []
      bool_out = True
      for i in list_keys:
          if i not in valid_keys:
              bool_out = False
              inv_keys.append(i)
  
      return bool_out, inv_keys