 According to Nefdt (2019b), the identity of a word is tied to its role in the sentence structure. In which case, “ain’t” and “isn’t” come out as the same word (at least in the singular use) but ‘watch’ (noun) and ‘watch’ (verb) do not. However, counterintuitively, his account might license the identity of words like ‘truck’ and ‘lorry’.