anchor.diff:      let
         lenStr = stringLength str;
         lenSuff = stringLength suff;
  -    in lenStr >= lenSuff &&
  -       substring (lenStr - lenSuff) lenStr str == suff;
  -
  +    in
  +    lenStr >= lenSuff
  +    && substring (lenStr - lenSuff) lenStr str == suff;
   
     # Convert a string to a list of characters (i.e. singleton strings).
     # For instance, "abc" becomes ["a" "b" "c"].  This allows you to,
  
positive.code: /* String manipulation functions. */
  
  let lib = import ./default.nix;
  
  inherit (builtins) length;
  
  in
  
  rec {
  
    inherit (builtins) stringLength substring head tail isString replaceStrings;
  
  
    # Concatenate a list of strings.
    concatStrings =
  
negative.code:   #    lib.getVersion pkgs.youtube-dl         ==> "2016.01.01"
    getVersion = x: (builtins.parseDrvName (x.name or x)).version;
  
    # Extract name with version from URL. Ask for separator which is
    # supposed to start extension.
    nameFromURL = url: sep:
  