Exegetical prowess: "binyan av" - learning the narrow case from a more general case. This category is added to the others: gezerah shavah, kal va-chomer, and hekesh. And the Gemara uses a baraita about disqualified blood as learned from a "binyan av" to probe whether it could then be applied to another binyan av. But that would mean deriving a law regarding that which was improperly done to that which was properly done, and that is difficult. And binyan av is determined to be a less used approach. Also, back to the halakhot about pouring the remaining blood - after the sprinkling - on the western side of the base of the external altar. And what might have been thought to do otherwise. But does the blood really need to be poured out there, when it doesn't bring about atonement or prevent it if it weren't done?