def add_external_ref(self, eid, etype, value):
if etype not in self.ETYPES:<EOL><INDENT>raise KeyError('<STR_LIT>'.format(self.ETYPES))<EOL><DEDENT>self.external_refs[eid] = (etype, value)<EOL>
Add an external reference.

        :param str eid: Name of the external reference.
        :param str etype: Type of the external reference, has to be in
            ``['iso12620', 'ecv', 'cve_id', 'lexen_id', 'resource_url']``.
        :param str value: Value of the external reference.
        :throws KeyError: if etype is not in the list of possible types.