-- twopower(n) = 2^(n+1)

twopower(Z)    = S(S(Z))
twopower(S(n)) = double2(twopower(n))

double2(S(S(n))) = S(S(S(S(double(n)))))

double(Z)    = Z
double(S(n)) = S(S(double(n)))
