anchor.diff:      toList
       types
       warnIf
  +    zipAttrsWith
       ;
     inherit (lib.options)
       isOption
  
positive.code:         }
        */
        byName = attr: f: modules:
          foldl' (acc: module:
                if !(builtins.isAttrs module.${attr}) then
                  throw ''
                    You're trying to declare a value of type `${builtins.typeOf module.${attr}}'
  
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; }]
  