src/Tools/Metis/metis_env.ML

always qualify NJ's old List.foldl/foldr in Isabelle/ML;

(* Metis-specific ML environment *)
nonfix ++ -- RL mem union subset;
val explode = String.explode;
val implode = String.implode;
val print = TextIO.print;
val foldl = List.foldl;
val foldr = List.foldr;