(Bamidbar 31:20) "and every garment and every vessel of skin": What is the intent of this? From (Vayikra 11:32) "or skin or sack," I know only of sack. Whence do I derive (the same for) every work of goats? (From the above.) Would you say that?
It follows a fortiori! (i.e., why do we need a verse?), viz.: If in dead-body tumah (our instance) the more stringent variety, every work of goats is likened to sack, then in the instance of sheretz (creeping thing) tumah, (that of Vayikra), the less stringent variety, how much more so should every work of goats be likened to sack! — Would you say that? Do we derive the less stringent from the more stringent to be more stringent with it?
Rather, why is "garment" mentioned in respect to dead body tumah? It follows a fortiori, viz.: If in sheretz, the less stringent variety, garment was likened to sack, how much more so (should this obtain) in dead-body tumah! Why, then, is "garment" mentioned in dead-body tumah? It is "extra" for the purpose of formulating an identity (gezeirah shavah), viz.: "Garment" is written here and "garment" is written elsewhere (Vayikra).
Just as here, every work of goats is likened to sack, so, there. And just as there, (the articles must be) spun and woven, (sack being spun and woven), so, there, spun and woven. To include the band, the belt, and the saddle-band of an ass, which are spun and woven. To exclude cords or ropes, which are not spun and woven.