Proof of Sklansky's theorem?

[Ralle]

I've been reading Sklansky's book The Theory of Poker, and
found it to be a very good book indeed. Especially I find
the chapters on semi-bluffing and raising to be excellent.

But, there is one thing that is missing. On page 17-18 he
presents what he calls The Fundamental Theorem of Poker.
Intuitively this theorem appears to be correct, but since
it is called a theorem, a proof should be included, or at
least available. Does anyone know where a proof can be
found?