Have a progressive stl exercise ?

[chavid]

I do not know what would be the right solution, but it is a problem that the stl exercise cannot be compiled until every hole is completed.

[stale[bot]]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.