items[ $key ] = $resolver; return $this; } public function register_fallback( Plain_Resolver_Base $resolver ): self { $this->fallback = $resolver; return $this; } public function has( string $key ): bool { return array_key_exists( $key, $this->items ); } public function get( $key, $fallback = null ) { return parent::get( $key, $fallback ?? $this->fallback ); } }