Imagine a class that shapes some blog posts. Each post is identified by a unique integer id and it has several properties including its tags. The tags of each post are actually represented as a string of tags separated by hashtag (#). Whenever we need the list of tags for a given post, we can call the allTags() helper method. Our goal is to write a stream pipeline that extracts from this list of tags a Map<String, List> containing for each tag (key) the list of posts (value).