When I read the trail on Generics the first time I fogged out. And when I read it a second time I fogged out again.
For me, key phrases include:
// Imagine this is one part of a large application
// modified by one programmer.
// ... and this is another, perhaps written
// by a different programmer
These phrases are a let out for me because I am a sole programmer working from home on a non-collaborative project. So the situation described with not apply.
Another key phrase is: "In any nontrivial software project, bugs are simply a fact of life". This tells me the trail has something to do with error trapping. This in turn raises two questions in my mind.
First, why is there nothing about error trapping or bug avoidance in the trail heading? Second, why is the first lesson which even mentions errors and bugs so incredibly esoteric? Why not have a basic lesson on bugs first, and then put the weird one in an appendix somewhere.
All in all I rank this trail below reference. For beginners I'd rate this a skip altogether trail.