is_sealed
Thm.is_sealed : string -> bool
Tests whether a theory has been sealed in the current session.
A sealed theory is locked against further change (see
Thm.mark_sealed for the guarantee). The theory
"min" is sealed at kernel bootstrap; a user theory becomes sealed
when Theory.export_theory writes it to disk or when the theory-load
machinery loads a pre-built copy into the session.
Failure
Never fails.