anchor.diff:  { package ? null
   , maintainer ? null
  +, path ? null
   }:
   
   # TODO: add assert statements
  
positive.code:                    (name: pkg: pkg)
                     pkgs;
  
    packageByName = name:
      let
          package = pkgs.lib.attrByPath (pkgs.lib.splitString "." name) null pkgs;
  
negative.code:       [ (packageByName package) ]
      else if maintainer != null then
        packagesWithUpdateScriptAndMaintainer maintainer
      else if path != null then
        packagesWithUpdateScript path
      else
        builtins.throw "No arguments provided.\n\n${helpText}";
  
  