Obtains the documentation for a link relation. Opens in a webbrowser
        window

def docsfor(self, rel):  # pragma: nocover
        '''Obtains the documentation for a link relation. Opens in a webbrowser
        window'''
        prefix, _rel = rel.split(':')
        if prefix in self.curies:
            doc_url = uritemplate.expand(self.curies[prefix], {'rel': _rel})
        else:
            doc_url = rel
        print('opening', doc_url)
        webbrowser.open(doc_url)