license information and detailed readme
basic propositional formula definitions

analogues of some definitions from "formulas.ml":
 * PFml type constructor for formulas of propositional logic
 * formula printing (with brackets)
 * formula destructors and Maybe versions of those
 * "eval", "overatoms", "onatoms" etc. (which are gained from
Monad/Foldable instances of PFml)