docs: function define_helpers_registrator(Klass) {
    Klass.registerHelper = function (name, helper, options) {
      // Scenario: registerHelper('foo', foo_helper[, foo_opts]);
      if (_.isString(name)) {
        Klass.prototype[name]             = helper;
        Klass.prototype.__helpers__[name] = {
          helper:   helper,
          options:  Object(options)
        };
        return;
      }
  
      // Scenario: registerHelper({ foo: foo_helper, ... });
      _.forEach(name, function (helper, name) {
        Klass.registerHelper(name, helper);
      });
    };
  }