lib.types.submoduleWith: Interoperate with older version of submoduleWith

          };

          binOp = lhs: rhs: {

            class =

              if lhs.class == null then rhs.class

              else if rhs.class == null then lhs.class

              else if lhs.class == rhs.class then lhs.class

              # `or null` was added for backwards compatibility only. `class` is

              # always set in the current version of the module system.

              if lhs.class or null == null then rhs.class or null

              else if rhs.class or null == null then lhs.class or null

              else if lhs.class or null == rhs.class then lhs.class or null

              else throw "A submoduleWith option is declared multiple times with conflicting class values \"${toString lhs.class}\" and \"${toString rhs.class}\".";

            modules = lhs.modules ++ rhs.modules;

            specialArgs =