Flag: Preserve the original formula with +cnf (for splitter heuristics).
CNF Caches
These caches are for subexpressions of the translated formula phi, to avoid expanding phi into a tree. We cannot use d_cnfCache for that, since it is effectively non-backtracking, and we do not know if a subexpression of phi was translated at the current level, or at some other (inactive) branch of the decision tree.