refined identity and ancestry: explicit draft flat, ids are unnamed, name consistency is checked for ancestry;
tuned history: renamed version to stage, disallow checkpoint/finish_thy on finished theories;
added display_names -- user-level presentation;
removed obsolete exists_name, names_of;
tuned;
List = FOL +
types list 1
arities list :: (term)term
consts Nil :: "'a list"
Cons :: "['a, 'a list] => 'a list"
end