I'm sorry, _I_ misunderstood the manual. I did not know that Dyalog dfns were always dyadic / ambivalent.
If I understand correctly then, in F {pow} G, G's left argument is _always_ the result of the power operator if G is true. Its right argument then is _always_ the result of the previous iteration. Is that correct?
If it is, I believe always ambivalent lambdas would be convenient (aside from possible implementation problems), but not at all essential; it's probably rare to check something about the previous iteration without comparing it to the current one. But wouldn't have been simpler to model them as ambivalent from the ground up?
Louis