more opportunistic string_of_clause, to make double-sure its Syntax.string_of_term uncheck phase does not crash, and thus bomb ambiguous input via failed composition of warning (e.g. HOL/Imperative_HOL/ex/Linked_List.thy: lemma merge_simps);
non-strict "Quiet" mode, to avoid odd crash of code_pred/after_qed due to ill-typed use of string_of_clause (e.g. in JinjaThreads/Common/Decl.thy of AFP/d1bb08f92ce5);