def queryURL(self, xri, service_type=None):
        """Build a URL to query the proxy resolver.

        @param xri: An XRI to resolve.
        @type xri: unicode

        @param service_type: The service type to resolve, if you desire
            service endpoint selection.  A service type is a URI.
        @type service_type: str

        @returns: a URL
        @returntype: str
        """
        # Trim off the xri:// prefix.  The proxy resolver didn't accept it
        # when this code was written, but that may (or may not) change for
        # XRI Resolution 2.0 Working Draft 11.
        qxri = toURINormal(xri)[6:]
        hxri = self.proxy_url + qxri
        args = {
            # XXX: If the proxy resolver will ensure that it doesn't return
            # bogus CanonicalIDs (as per Steve's message of 15 Aug 2006
            # 11:13:42), then we could ask for application/xrd+xml instead,
            # which would give us a bit less to process.
            '_xrd_r': 'application/xrds+xml',
            }
        if service_type:
            args['_xrd_t'] = service_type
        else:
            # Don't perform service endpoint selection.
            args['_xrd_r'] += ';sep=false'
        query = _appendArgs(hxri, args)
        return query