'Add a single, new feature.
Arguments:
feature -- The namespace of the supported feature.'

def add_feature(self, feature):

    if (feature not in self._features):
        self._features.add(feature)
        feature_xml = ET.Element(('{%s}feature' % self.namespace))
        feature_xml.attrib['var'] = feature
        self.xml.append(feature_xml)
        return True
    return False
