That's interesting thing to see in GNU APL. Negative exponents are obviously tricky to do, but what about using boolean functions? It's a quite convenient way to succinctly express iteration. But right now GNU APL gives somewhat unexpected results:
({2+⍵÷2}⍣≡) 9
9
({2+⍵÷2}⍣=) 9
VALENCE ERROR
(λ1⍣=)9
^ ^
In both cases Dyalog's answer would be 4, i.e. a fixed point of {2+⍵÷2}.
-k
