code: func (conf FilterConfigurator) Add(f Filter) FilterConfigurator {
  	conf.apply(func(fc []Filter) []Filter {
  		return conf.addFilter(f, fc)
  	})
  	return conf
  }
docs: // Add the given filter in the second-to-last position in the filter chain.
  // (Second-to-last so that it is before ActionInvoker)