  Anderson's interlayer tunneling model can account for up to 1% of the
condensation energy of Tl_2Ba_2CuO_y. Here we account for the remaining 99%. We
predict an in-plane kinetic energy gain of 1 to 3 meV per planar oxygen in the
optimally doped material when it goes superconducting. It is suggested that the
effect may be most easily detected in underdoped dirty samples.
