The affixes yat and ḍhak comes in the sense of 'what bears it' after the word dhura in the Accusative -2nd case in construction.,
Thus धुरं वहति = धुर्यः (formed by यत्) the lengthening ordained by 8.2.77 is prevented by 7.2.79 because धुर is Bha-stem, or धौरेयः (formed by डक् 7.1.2) 'a beast of burden'. This sutra might have stood as धुरो ढक् च, for यत् would have been read into it from 4.4.75.,