The Yerushalmi (pesachim 58 in vilna ed) cites the hekesh of matzah to chameitz to explain why women are obligated in the mitzvah of matzah. The simple pshat in the hekesh that I remember from the Bavli is that the limud is needed to overcome the exemption of zman gerama. The Yerushalmi, however, cites the mishna that exempts women from mitzvos which are zman gerama as a contradiction to the hekesh – very strange, because the whole point is that the hekesh is the exception that proves the rule. The Yerushalmi answers that matzah is different because it is an aseh “she’hi ba’ah m’koach lo ta’aseh”, which stems from a lav. What does that mean?
Kiddushin 34 gives three examples of mitzvos which are not zman gerama which women are therefore obligated in: hashavas aveidah, ma’akeh, and shiluach hakan. Tosfos points out that in each of these three cases there is a potential lav that is violated along with the aseh: there is an issur of lo tasim damim for not building a ma’akeh, an issur of lo tuchal l’hitalem for not returning a lost object, an issur of lo tikach ha’aim by shiluach hakan. Whether or not women are obligated in the mitzvos aseh would seem irrelevant, as women would be obligated to avoid the lavim. Ramban answers that in certain cases the lav is not an independent violation, but is just an extension of the aseh associated with it, an added stringency to enhance the aseh. Had women not been obligated in the aseh, the lav would not apply either.