code: func (conf FilterConfigurator) Insert(insert Filter, where When, target Filter) FilterConfigurator {
  	if where != BEFORE && where != AFTER {
  		panic("where must be BEFORE or AFTER")
  	}
  	conf.apply(func(fc []Filter) []Filter {
  		return conf.insertFilter(insert, where, target, fc)
  	})
  	return conf
  }
docs: // Insert a filter into the filter chain before or after another.
  // This may be called with the BEFORE or AFTER constants, for example:
  //   revel.FilterAction(App.Index).
  //     Insert(MyFilter, revel.BEFORE, revel.ActionInvoker).
  //     Insert(MyFilter2, revel.AFTER, revel.PanicFilter)