@pre  0 <= l && u < |a| && sorted(a, 0, |a| - 1)
@post rv <-> exists ix. (l <= ix && ix <= u && a[ix] = e)
bool BinarySearch(int[] a, int l, int u, int e) {
	if (l > u) 
		return false;
	else {
		int m := (l + u) div 2;
		if (a[m] = e) 
			return true;
		else if (a[m] < e) 
			return BinarySearch(a, m + 1, u, e);
		else 
			return BinarySearch(a, l, m - 1, e);
	}
}