solution to depth-first-search

I've offered a depth-first-search solution on the '99 problems' page. Unfortunately, due to issues (https://github.com/ocaml/opam/issues/1410) installing opam-lib, haven't been able to see how it renders (but it should be fine).
I appreciate you pass `t` but I am wondering whether it is the right value of t to pass.  I think it would be better that `t` is the distance to the initial node (something that is easy to describe in the statement).