// net.minecraft.commands.arguments.NbtPathArgument.MatchElementNode

public void getOrCreateTag(Tag tag, Supplier<Tag> supplier, List<Tag> list) {
    MutableBoolean anyMatch = new MutableBoolean();
    if (tag instanceof ListTag) {
        ListTag listTag = (ListTag) tag;
        listTag.stream().filter(this.predicate).forEach(t -> {
            list.add(t);
            anyMatch.setTrue();
        });
        if (anyMatch.isFalse()) {
            CompoundTag parent = this.pattern.copy();
            
            // If none of the elements of `listTag` matches `this.predicate`, a compound tag `parent` is added to `listTag`.
            listTag.add(parent);

            list.add(parent);
        }
    }
}

// net.minecraft.nbt.ListTag

public void add(int index, Tag tag) {
    if (!this.addTag(index, tag)) {
        // Since `tag` is a compound and this list is not a compound list in this case, `this.addTag(index, tag)` returns false and `UnsupportedOperationException` is thrown.
        throw new UnsupportedOperationException(String.format("Trying to add tag of type %d to list of %d", tag.getId(), this.type));
    }
}