More on the Budget Crisis

According to press accounts (such as the one here from the San Francisco Chronicle), the budget deal between the Legislature and the Governor involves (among other things) a "rainy day fund," that would be collected by the state and used only for specified purposes. Indeed, one of the final sticking points in the negotiations was the issue of when the "rainy day fund" could be used. Here's the constitutional issue: given that one legislative body generally can't bind its successors, how does a statute such as the annual Budget Act restrict the Legislature's future ability to tap into the "rainy day fund" for whatever purposes the Legislature then deems appropriate? Or does the deal creating the "rainy day fund" involve submitted a constitutional amendment to the electorate that would create the fund and establish the conditions under which it could be tapped?