libexact is a software library for solving combinatorial exact
covering problems. It implements essentially the backtrack algorithm
and the dancing links data structure described in "Donald E. Knuth,
Dancing Links, Millennial Perspectives in Computer Science (J.
Davies, B. Roscoe, and J. Woodcock, Eds.), Palgrave, Basingstoke,
England, 2000, pp. 187-214" (preprint).