@staticmethod
def _key2identifier(key):
    """Convert keys to valid identifiers.

        This method could be redefined in sub-classes to change this feature.
        This function must return a valid identifier or None if the conversion
        is not possible.

        """
    from xotl.tools.keywords import suffix_kwd
    from xotl.tools.string import slugify
    from xotl.tools.validators import is_valid_identifier
    res = key if is_valid_identifier(key) else slugify(key, '_')
    return suffix_kwd(res)
