anchor.diff:          fold' = n:
             if n == len
             then nul
  -          else op (elemAt list n) (fold' (add n 1));
  +          else op (builtins.elemAt list n) (fold' (add n 1));
         in fold' 0
       else op: nul:
         let fold' = list:
  
positive.code:         foldl' = n:
            if n == minus1
            then nul
            else op (foldl' (sub n 1)) (elemAt list n);
        in foldl' (sub (length list) 1)
      else op:
        let foldl' = nul: list:
  
negative.code: # General list operations.
  
  rec {
    inherit (builtins) head tail length isList add sub lessThan;
  
  
    # 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];
  
  
    # "Fold" a binary function `op' between successive elements of
    # `list' with `nul' as the starting value, i.e., `fold op nul [x_1
  