code: function (hoodie, db, callback) {
  
      var index = {
        map: function (doc) {
          if (doc.source && doc.target && doc.subject) {
            var subject = doc.subject.join(',');
            emit([doc.source, subject], { userId: doc.target.split('/').pop(), subject: subject });
          }
        },
        reduce: function (key, value, rereduce) {
          if (!rereduce)
            return value;
        }
      };
  
      db.addIndex('by_subscribers', index, function (err) {
        if (err) {
          return callback(err);
        }
  
        return callback();
      });
    }
docs: PubSub addIndexSubscribers