q = e div 2
  r = e mod 2
then e = 2q+r, and r could be 0 or 1.

If r=0:

    b^e = (b^q)^2

If r=1:

    b^e = (b^q)^2 * b

base case: b^0 = 1.

q = 2/2 = 1
r = 2mod2 = 0

r=0, therefore 2^2 = 2^1^2

pow :: Integer -> Integer -> Integer
pow b e
    | e == 0 = 1
    | r == 0 = pow (pow b q) 2
    | r == 1 = pow (pow b q) 2 * b
  where
    (q, r) = divMod e 2