So I'm still without an example that really tests the key feature of the new system: I need to find a proposition A such that ~~A^o is provable (in the empty contex) but ~~A is not, where ^o is the double-negation translation
(A => B)^o = ~~A => ~~B
(A v B)^o = ~~A v ~~B
(A ^ B)^o = ~~A ^ ~~B
p^o = p
T^o = T
F^o = F