I was annoyed at the way that Duolingo chooses the lesson when you decide to practice.
What it does is that it chooses the first skill that does not have a full strength bar, starting from the bottom of the tree, and makes you practice that.
So unless you practice a lot, you end up practicing the lower part of the tree while the upper part remains neglected.
This small extension fixes this by choosing the skill with the lowest strength value, regardless of its position in the tree, and makes you practice that.