// TIP: Try re-using reduce() here.
    var test = true;
    if (iterator === undefined) {
      for (var key in collection) {
        if (! collection[key] && test) {
          return false;
        }
      }
    } else {
      for (var key in collection) {
        if (! iterator(collection[key]) ) {
          return false;
        }
      }
    }
    return test;