add_tagThm.add_tag : tag * thm -> thm
Adds oracle tags to a theorem.
A call to add_tag(tg,th) returns a th' such
that calling Thm.tag(th') returns the tag that is the merge
of the tag associated with th (if any) and
tg.
Never fails.
If an oracle implementation wishes to record additional information about the oracle mechanisms that have contributed to the ‘proof’ of a theorem (perhaps the use of existing HOL theorems that will have their own tags), then this function can be used to add that record.