@NotNull
        @Override
        public Iterator<TypeMigrationUsageInfo> getIn(TypeMigrationUsageInfo n) {
          final HashSet<Pair<TypeMigrationUsageInfo, PsiType>> rawNodes = myRootsTree.get(n);
          if (rawNodes == null) {
            return Collections.emptyIterator();
          }
          final List<TypeMigrationUsageInfo> in =
            ContainerUtil.map(rawNodes, pair -> pair.getFirst());
          return in.iterator();
        }
@NotNull
        @Override
        public Iterator<TypeMigrationUsageInfo> getIn(TypeMigrationUsageInfo n)