code: function(config, stream, extras, callback) {
  
      // Finish the stream so we get a full array of files, that
      // we can use to figure out whether add prev/next placeholders.
      streamHelpers.finishWithFiles(stream, function(files) {
  
        // loop through each file and get the title of the heading
        // as well as the relative file path.
        _.each(files, function(file, i) {
  
          var nav = {};
  
          // if there was a file before this one, add placeholders
          if(files[i-1]) {
            file.prev = files[i-1];
            nav.prev = { label: "MBINSERTPREVLABEL", href: "MBINSERTPREVHREF" }
          }
  
          // if there is a file after this one, add placeholders
          if(files[i+1]) {
            file.next = files[i+1];
            nav.next = { label: "MBINSERTNEXTLABEL", href: "MBINSERTNEXTHREF" }
          }
  
          _.set(file, "pageLocals.navigation", nav);
          _.set(file, "layoutLocals.navigation", nav);
  
        });
  
        // create new stream from the files
        stream = streamHelpers.streamFromArray(files);
  
        callback(null, config, stream, extras);
  
      });
    }
docs: On load we add placeholders, to be filled in later in the build process when we have the correct filenames.