Parse word substitutions
        :param element: The XML Element object
        :type  element: etree._Element

def _parse_substitutions(self, element):
        """
        Parse word substitutions
        :param element: The XML Element object
        :type  element: etree._Element
        """
        subs = element.findall('sub')

        for sub in subs:
            self.agentml.set_substitution(attribute(sub, 'word'), sub.text)