nicer error message, indicating which type is empty (relying on metatheoretical completeness proof here)
src/Tools/Code
src/Tools/jEdit
src/Tools/WWW_Find
src/HOL/Mirabelle
src/HOL/Mutabelle
src/HOL/Library/Sum_of_Squares
src/HOL/Tools/ATP
src/HOL/Tools/Predicate_Compile
src/HOL/Tools/SMT
src/HOL/TPTP