anchor.diff:      showFiles
       showOption
       unknownModule
  -    literalExpression
       ;
   
     showDeclPrefix = loc: decl: prefix:
  
positive.code:      'opts' is a list of modules.  Each module has an options attribute which
       correspond to the definition of 'loc' in 'opt.file'. */
    mergeOptionDecls =
     let
      coerceOption = file: opt:
        if isFunction opt then setDefaultModuleLocation file opt
        else setDefaultModuleLocation file { options = opt; };
     in loc: opts:
      foldl' (res: opt:
        let t  = res.type;
            t' = opt.options.type;
  
negative.code:                 }
                else
                  let
                    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 `${(lib.head optionDecls).options.type.description or "<no description>"}' does not support nested options.\n${
  