anchor.diff:      if list == []
       then nul
       else op (head list) (fold op nul (tail list));
  -
       
     # Concatenate a list of lists.
     concatLists =
  
positive.code:   catAttrs = attr : l : fold ( s : l : if (__hasAttr attr s) then [(__getAttr attr s)] ++ l else l) [] l;
  
    flattenSet = set : map ( attr : __getAttr attr set) (__attrNames set);
  
  # Marc 2nd proposal: (not everything has been tested in detail yet..)
  # One example showing how to use mandatory dependencies [1], default flags [2], ..
  
negative.code:     #    alternativeAddOn  = { cfgOption = "--enable-add-ons-alt"; blocks = ["addOns", "addOns2"]; };
      #    justAOption =  { };
      #  };
      #  co = chooseOptionsByFlags flagDescr ["libpng", "libjpg"] args;
  
      #in args.stdenv.mkDerivation {
  
  