[Concerning] a sheet that contracted midras tumah and was [then] made into a curtain, it becomes tahor from midras [tumah], but it is tamei [due to] contact with a midras. R’ Yose said: Why, which midras did this [sheet] touch? Rather [it contracts contact-tumah] only if the zav himself touches it, [rendering] it tamei [on account of] the zav’s touching.