private static <T> void mergeMaps(@NotNull Map<T, List<DomElementProblemDescriptor>> accumulator, @Nullable Map<T, List<DomElementProblemDescriptor>> toAdd) {
    if (toAdd == null) {
      return;
    }

    for (Map.Entry<T, List<DomElementProblemDescriptor>> entry : toAdd.entrySet()) {
      accumulator.computeIfAbsent(entry.getKey(), __ -> new SmartList<>()).addAll(entry.getValue());
    }
  }
private static <T> void mergeMaps(@NotNull Map<T, List<DomElementProblemDescriptor>> accumulator, @Nullable Map<T, List<DomElementProblemDescriptor>> toAdd)