anchor.diff:  
   rec {
   
  -  inherit (builtins) head tail length isList elemAt concatLists filter elem genList;
  +  inherit (builtins)
  +    head
  +    tail
  +    length
  +    isList
  +    elemAt
  +    concatLists
  +    filter
  +    elem
  +    genList;
   
   
     # Create a list consisting of a single element.  `singleton x' is
     # sometimes more convenient with respect to indentation than `[x]'
     # when x spans multiple lines.
  -  singleton = x: [x];
  +  singleton = x: [ x ];
   
   
     # "Fold" a binary function `op' between successive elements of
  
positive.code:       let
          len = length list;
          imap' = n:
            if n == len
              then []
              else [ (f (n + 1) (elemAt list n)) ] ++ imap' (n + 1);
        in imap' 0;
  
  
    # Map and concatenate the result.
  
negative.code:   # Reverse the order of the elements of a list.
    reverseList =
      if builtins ? genList then
        xs:
        let
          l = length xs;
        in
        genList (n: elemAt xs (l - n - 1)) l
      else
        fold (e: acc: acc ++ [ e ]) [];
  
  