def url_to_prefix(self, id):
        '''url_to_prefix

        High-level api: Convert an identifier from `{namespace}tagname` notation
        to `prefix:tagname` notation. If the identifier does not have a
        namespace, it is assumed that the whole identifier is a tag name.

        Parameters
        ----------

        id : `str`
            Identifier in `{namespace}tagname` notation.

        Returns
        -------

        str
            Identifier in `prefix:tagname` notation.
        '''

        ret = re.search('^{(.+)}(.+)$', id)
        if ret:
            return self.urls[ret.group(1)] + ':' + ret.group(2)
        else:
            return id