Unlike the modal and aspectual markers which are used to form the other complex negative particles, "su" is not attested as a free morpheme elsewhere; however, it is likely related to the optional final "-s" of the modal verb "-ahate(s)" 'to be unable', which appears to be derived historically from "te" having fused onto the verb "-ahan" 'to be able'. When used with a final "-s", as in (25b) compared with (25a), this modal verb encodes the same meanings expressed by "tesu":