anchor.diff:  
   let
     inherit (builtins) head tail length;
  -  inherit (lib.trivial) and;
  +  inherit (lib.trivial) id;
     inherit (lib.strings) concatStringsSep sanitizeDerivationName;
  -  inherit (lib.lists) foldr foldl' concatMap concatLists elemAt;
  +  inherit (lib.lists) foldr foldl' concatMap concatLists elemAt all;
   in
   
   rec {
  
positive.code:   */
    mapAttrsRecursiveCond = cond: f: set:
      let
        recurse = path: set:
          let
            g =
              name: value:
              if isAttrs value && cond value
                then recurse (path ++ [name]) value
                else f (path ++ [name]) value;
          in mapAttrs g set;
      in recurse [] set;
  
  
  
negative.code:        foldAttrs (n: a: [n] ++ a) [] [{ a = 2; } { a = 3; }]
         => { a = [ 2 3 ]; }
    */
    foldAttrs = op: nul:
      foldr (n: a:
          foldr (name: o:
            o // { ${name} = op n.${name} (a.${name} or nul); }
          ) a (attrNames n)
      ) {};
  
  
    /* Recursively collect sets that verify a given predicate named `pred'
  