unsigned long peak(unsigned long nel, int (*less)(unsigned long i, unsigned long j)){
        unsigned long left = 0, right = nel-1, x, ans;
        while (left<right){
            x = left + (right-left)/2;
            if (less(x+1, x)) right = x;
            else left = x+1;
        }
        if(less(right, left)) ans = left;
        else ans = right;
        return ans;
}