@staticmethod
def _schema_add_empty(dict_schema):
    """Add `empty: False`` specification for all fields without
        ``empty`` specification

        Parameters
        ----------
        dict_schema : dict
            Schema of ``field: specification`` items, for which each
            specification is a dict itself.
        """
    for term, rules in dict_schema.items():
        if 'empty' not in rules.keys():
            rules['empty'] = False
    return dict_schema
