private val mapping = HashMap<FirElement, Optional<FirElement>>()
        private var unmappedCount = statements.size

        private val stack = ArrayDeque<FirElement>()

        operator fun get(fir: FirElement): FirElement? {
            return mapping[fir]?.getOrNull()
        }

        override fun visitElement(element: FirElement) {
            withElement(element) {