Haha, edited while edited. Thanks, looks like I got it.

Hmm, it looks like { exp cost, { pre-req1, pre-req2, ... }, { choose one of these1, choose one of these2 }.

The third {} appears to be used only by the gun specialisations (choose HC / AC / Laser). I wonder if we can put other perks in here as well. E.g. choose between Criticals_1 or Hard_to_Kill.