// efficient.
type factsTable struct {
	// unsat is true if facts contains a contradiction.
	//
	// Note that the factsTable logic is incomplete, so if unsat
	// is false, the assertions in factsTable could be satisfiable
	// *or* unsatisfiable.
	unsat      bool // true if facts contains a contradiction
	unsatDepth int  // number of unsat checkpoints

	facts map[pair]relation // current known set of relation

        //  c. There is no path from B to A in the cycles graph (but there may
        //     be a path from A to B).
        //
        // So check the legality of the edge contraction by checking if any of
        // the n^2 pairs of resource variable operations are forbidden.
        if (unsafe_resource_deps_.contains(
                {resource_var_from, resource_var_to})) {