-- | Extraction of symbols from a signature
symOf :: Sign.Sign -> Set.Set Symbol
symOf x = Set.fold (\ y -> Set.insert Symbol {symName = y}) Set.empty $
           Sign.items x
symOf :: Sign.Sign -> Set.Set Symbol
symOf x = Set.fold (\ y -> Set.insert Symbol {symName = y}) Set.empty $
           Sign.items x
symOf x = Set.fold (\ y -> Set.insert Symbol {symName = y}) Set.empty $
           Sign.items x