Is lemma defined for adverbs? For example, what is the lemma of diagonally? An even more difficult example would be "What is the lemma of nationally?" Is it nationally, national ,or nation? Or lemma is simply the word itself, for adverbs?
In wikipedia it says that lemma is the canonical form chosen from a lexeme (a set of forms carrying the same meaning). Is it true if I say that since diagonally and diagonal doesn't carry the same meaning (even it changes word class), then the two of them is not part of the same lexeme, and hence one can't be the lemma of another?
If that's the case, then is there any example of an adverb that has a different lemma?
I've read this question: What is the notion of lemma? but it doesn't seem to answer my question directly (and there is no answer in that question either).