This Git repository contains two things: a copy of the Darcs repository for the LogicT monad transformer a folder called introduction/.
The LogicT monad is "[a]n adaptation of the two-continuation implementation found in the paper Backtracking, Interleaving, and Terminating Monad Transformers.
The file introduction/adapting-paper-to-logict.hs contains my adaptation of the code in that paper to run in the LogicT monad, with explanatory comments.
The folder introduction/for-paper,by-its-authors/ contains the code they wrote for the paper. I haven't worked through it, but I did refer to it a few times when my own adaptations weren't working.