use rand::{Rand, Rng, XorShiftRng, self};
pub fn vec<T>(size: usize, start: usize) -> Option<Vec<T>> where T: Rand {
if size > start + 1 {
let mut rng: XorShiftRng = rand::thread_rng().gen();
Some((0..size).map(|_| rng.gen()).collect())
} else {
None
}
}