To do & bugs
------------

XXX: name is not great

XXX: is assert functionality worthwhile?

XXX: CustomValidator is poor name

TODO: unify naming scheme

TODO: how to handle negation

XXX: is there a better way to make_list?

XXX: everything in the one namespace?

TODO: email, url, etc. validators

TODO: toRegex or ToRegexSub (for subpatterns) & regex transformations
