An invocation fetch thy name searches through the currently loaded
theory segments in an attempt to find a theorem, axiom, or definition
stored under name in theory thy.
FAILURE
If the specified theorem, axiom, or definition cannot be located.
EXAMPLE
- DB.fetch "bool" "NOT_FORALL_THM";
> val it = |- !P. ~(!x. P x) = ?x. ~P x : thm