code: func NewHandlerFunc(allow, deny func(http.ResponseWriter, *http.Request), acl ACL) (*HandlerFunc, error) {
  	if allow == nil {
  		return nil, errors.New("whitelist: allow cannot be nil")
  	}
  
  	if acl == nil {
  		return nil, errors.New("whitelist: ACL cannot be nil")
  	}
  
  	return &HandlerFunc{
  		allow:     allow,
  		deny:      deny,
  		whitelist: acl,
  	}, nil
  }
docs: // NewHandlerFunc returns a new basic whitelisting handler.