Function tribonacci 

pub fn tribonacci(n: u64) -> u128

n-th Tribonacci number (T_0 = 0, T_1 = 0, T_2 = 1, T_n = T_{n-1} + T_{n-2} + T_{n-3}).

Examples

use mathverse_core::integer::tribonacci;

assert_eq!(tribonacci(0), 0);
assert_eq!(tribonacci(3), 1);
assert_eq!(tribonacci(4), 2);