I love the solo on "Zanzibar," but it's not very technically demanding. There would have been no reason to overdub it. Someone here years ago said something very cogent about Sergio's playing, something like "Sergio may not be the best pianist in the world, but he's one of the smartest." I agree absolutely--his solos are always models of construction and clear, concise statements. Nothing overtly flashy, in the Tatum or Eldar senses, but always very solid.