docs: function serve(server, req, callback) {
  		source.getTile(req.z, req.x, req.y, function(err, buffer) {
  			if (err) return callback(err);
  			var headers = {'Content-Type': 'application/x-protobuf'};
  			if (options.compression === 'gzip') {
  				headers['Content-Encoding'] = 'gzip';
  			}
  			callback(null, buffer, headers);
  		});
  	}