-- 2^(y^2) ==> 4^y
normaliseExp b@(S [P [_]]) (S [e@(P [_])]) = S [P [reduceExp (E b e)]]
normaliseExp b@(S [P [_]]) (S [e@(P [_])]) = S [P [reduceExp (E b e)]]
normaliseExp b@(S [P [_]]) (S [e@(P [_])]) = S [P [reduceExp (E b e)]]