anchor.diff:                  }
                 else
                   let
  -                  firstNonOption = findFirst (m: !isOption m.options) "" decls;
                     nonOptions = filter (m: !isOption m.options) decls;
                   in
                   throw "The option `${showOption loc}' in module `${(lib.head optionDecls)._file}' would be a parent of the following options, but its type `${(lib.head optionDecls).options.type.description or "<no description>"}' does not support nested options.\n${
  
positive.code:     concatStringsSep
      elem
      filter
      findFirst
      foldl'
      getAttrFromPath
      head
  
negative.code:      'opts' is a list of modules.  Each module has an options attribute which
       correspond to the definition of 'loc' in 'opt.file'. */
    mergeOptionDecls =
     loc: opts:
      foldl' (res: opt:
        let t  = res.type;
            t' = opt.options.type;
  