gracefully handle the case where a constant is partially or not instantiated at all, as may happen when reconstructing Metis proofs for polymorphic type encodings
(*
To update:
cp /home/lcp/isabelle/Repos/HOL/Auth/{Message.thy,Message_lemmas.ML,Event.thy,Event_lemmas.ML,Public.thy,Public_lemmas.ML,NS_Public.thy} .
*)
use_thy "NS_Public";