As we now, syllable codas -ow, -ow', -uw, and -uw' are impossible. Out of the remaining 121 codas, 118 do occur (the rarest occurring is -Irgh). The 3 codas that have not been observed in canon yet are -ew', -Iw', and -Iy'. Are they actually impossible or we just have not seen words with them yet?