you apply another monoid operation. one possible one is to return first or last element. makes sense in layered configuration of, for example, a text editor, w

you apply another monoid operation. one possible one is to return first or last element. makes sense in layered configuration of, for example, a text editor, where you might override a colour. another possible one is to return error on duplicate. makes sense in flat configuration of, for example, a build system. your application knows which operation fits its intended structure. your application documents the behaviour, just as it normally would.