anchor.diff:          foldl' = n:
             if n == minus1
             then nul
  -          else op (foldl' (sub n 1)) (elemAt list n);
  +          else op (foldl' (sub n 1)) (builtins.elemAt list n);
         in foldl' (sub (length list) 1)
       else op:
         let foldl' = nul: list:
  
positive.code: # General list operations.
  
  rec {
    inherit (builtins) elemAt 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
  
negative.code:         fold' = n:
            if n == len
            then nul
            else op (builtins.elemAt list n) (fold' (add n 1));
        in fold' 0
      else op: nul:
        let fold' = list:
  