anchor.diff:          }
         */
         byName = attr: f: modules:
  -        foldl' (acc: module:
  +        zipAttrsWith (n: concatLists)
  +          (map (module:
                 if !(builtins.isAttrs module.${attr}) then
                   throw ''
                     You're trying to declare a value of type `${builtins.typeOf module.${attr}}'
  
positive.code:     toList
      types
      warnIf
      ;
    inherit (lib.options)
      isOption
  
negative.code:                   this option by e.g. referring to `man 5 configuration.nix'!
                  ''
                else
                  mapAttrs (n: f module) module.${attr}
                ) modules);
        # an attrset 'name' => list of submodules that declare ‘name’.
        declsByName = byName "options" (module: option:
            [{ inherit (module) _file; options = option; }]
  