  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.
