Second level nesting for permalink config

I want to have a flat hierarchy for links… I know I can create a permalink config for everything under library which is a top level dir inside of content but I want to create the same flat permalink config for nested folders inside of library as well.

As an example:
This works: content/library/ =>
This does not work: content/library/posts/ =>

How do I make the second one work?

Additional note:
On the forum, this is the only thing I could find, which addresses my problem but no solution is provided: Permalink to subsections in config