Worth while papers to read for pkg manager/resolver background (mirrored locally)-
http://www.pkgcore.org/~ferringb/misc-pdfs/model.pdf
http://www.pkgcore.org/~ferringb/misc-pdfs/edos-wp2d1.pdf

prolog state machine implementation (general approach is similar to resolvers actions)-
http://www.vanx.org/archive/wam/wambook.pdf