"Dirty Willie", composition credited to Marty Sheller and recorded by Mongo Santamaria, seems to have an introductory piano vamp and then blowing changes both based on "Impressions ("So What" changes), but the head between the intro and blowing seems to be based on "Bu's Delight" (which is not based on "So What" changes). Has this been written about anywhere? I don't have the liner notes to the Santamaria albums on which "Dirty Willie" has been released. I'd welcome any comments people have about this track.