Add a small exercise for the user

Author: nicolasstucki

src/main/g8/src/main/scala/Main.scala

@@ -35,6 +35,7 @@ object Main {
   def powerCode(n: Int, x: Expr[Double]): Expr[Double] =
     if (n == 0) 1.0.toExpr // toExpr lifts 1.0 to '(1.0)
     else if (n == 1) x // optimization to not generate x * 1
+    else if (n < 0) throw new Exception("Negative powers not implemented. Left as a small exercise. Dont be shy, try it out.")
     else if (n == 2) '(~x * ~x) // optimization to not generate { val y = x; y * y }
     else if (n % 2 == 1)  '{ ~x * ~powerCode(n - 1, x) }
     else '{ val y = ~x * ~x; ~powerCode(n / 2, '(y)) }