added "plural : 'a -> 'a -> 'b list -> 'a" for convenient error msg construction
added "plural : 'a -> 'a -> 'b list -> 'a" for convenient error msg construction

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle