anchor.diff:        defnsByName' = byName "config" (module: value:
             [{ inherit (module) file; inherit value; }]
           ) configs;
  -    in
  -    (flip mapAttrs declsByName (name: decls:
  -      # We're descending into attribute ‘name’.
  +
  +      resultsByName = flip mapAttrs declsByName (name: decls:
  +        # We're descending into attribute ‘name’.
           let
             loc = prefix ++ [name];
             defns = defnsByName.${name} or [];
  
positive.code:             in
                throw "The option `${showOption loc}' in `${firstOption._file}' is a prefix of options in `${firstNonOption._file}'."
            else
              mergeModules' loc decls defns
        ))
      // { _definedNames = map (m: { inherit (m) file; names = attrNames m.config; }) configs; };
  
    /* Merge multiple option declarations into a single declaration.  In
       general, there should be only one declaration of each option.
  
negative.code:         };
        };
  
        merged =
          let collected = collectModules
            (specialArgs.modulesPath or "")
            (modules ++ [ internalModule ])
            ({ inherit lib options config; } // specialArgs);
          in mergeModules prefix (reverseList collected);
  
        options = merged.matchedOptions;
  
        config = mapAttrsRecursiveCond (v: ! isOption v) (_: v: v.value) options;
  
        checkUnmatched =
          if config._module.check && merged.unmatchedDefns != [] then
            let inherit (head merged.unmatchedDefns) file prefix;
            in throw "The option `${showOption prefix}' defined in `${file}' does not exist."
          else null;
  
        result = builtins.seq checkUnmatched {
          inherit options;
          config = removeAttrs config [ "_module" ];
          inherit (config) _module;
  