'Sets common named extensions to private attributes and creates a list
of critical extensions'

def _set_extensions(self):

    self._critical_extensions = set()
    for extension in self[u'single_extensions']:
        name = extension[u'extn_id'].native
        attribute_name = (u'_%s_value' % name)
        if hasattr(self, attribute_name):
            setattr(self, attribute_name, extension[u'extn_value'].parsed)
        if extension[u'critical'].native:
            self._critical_extensions.add(name)
    self._processed_extensions = True
