load_thy/after_load: explicit check of parent theories, which might have failed to join proofs -- avoid uninformative crash via Graph.UNDEF;
(* Classical Higher-order Logic -- batteries included *)
use_thys ["Library", "List_Prefix", "List_lexord", "Sublist_Order",
"Code_Char_chr", "Code_Integer", "Efficient_Nat", "Executable_Set", "Code_Prolog"];