That could've used an example of debugging recursion to show how useful the repl is. I knew about the repl and have used it to find duplicate packages for example, but what it doesn't help with is finding out how stuff was added to environment.systemPackages and, most importantly, why.
The most glaring omission in nix is the lack of a debugger with conditional breakpoints. Nix is interpreted, is it not? Shouldn't it be possible to have breakpoints?