MESFET Current Source (Sedra & Smith)

[Caspar Magi]

I am referring to the Textbook Sedra and Smith, the section on GaAs amplifiers.
I am referring to the Bootstrapping of a MESFET current source.
More specifically its small-signal equivalent circuit.
In that section alpha and subsequently the output resistance is derived.
I tried several times to derive the alpha but I never got the result obtained in the book.
I am using the 3rd edition, page 465 equation 6.152

I would appreciate if anyone could provide me with the derivation.

Thanks a lot!