todo
simpler NNF, with polarity-dependent elimination of equivalence
done right
additional functions in DBEnv
prepare for renaming in CNF
De Bruijn manipulations in Formulas and ScopedTerms
progress in formula renaming, with a cleaner interface in Skolem and some more code in Cnf
definitional CNF implemented, based on an estimation of the number
of clauses implemented with dynamic programming.
merge from branch better_cnf