Complete "Born's rule" from "environment-assisted invariance" in terms   of pure-state twin unitaries
  Zurek's derivation of the Born rule from envariance (environment-assisted invariance) is tightened up, somewhat generalized, and extended to encompass all possibilities. By this, besides Zurek's most important work, also the works of 5 other commentators of the derivation are taken into account, and selected excerpts commented upon. All this is done after a detailed theory of twin unitaries, which are the other face of envariance. 
Fedor Herbut