code: function(matches, callback) {
        var inflight = matches.length;
  
        // return early if there is no fetching to be done
        if (!inflight) {
          return callback();
        }
  
        // wait for all subrequests to return
        var done = function() {
          if (--inflight === 0) {
            callback();
          }
        };
  
        // start fetching all subrequests
        var m, req, url;
        for (var i = 0; i < inflight; i++) {
          m = matches[i];
          url = m.url;
          req = this.cache[url];
          // if this url has already been requested, skip requesting it again
          if (!req) {
            req = this.xhr(url);
            req.match = m;
            this.cache[url] = req;
          }
          // wait for the request to process its subrequests
          req.wait(done);
        }
      }
docs: build a mapping of url -> text from matches