Fibonacci sequence and Fibonacci numbers are extremely famous. The sequence begins with 0, 1 and each subsequent number is the sum of the two numbers before it. Thus fib(0) = 0, fib(1) = 1, fib(n) = fib(n-1) + fib(n-2). On the lines of Fibonacci numbers, we will define a Tribonacci sequence as one that starts with 0, 0, 1 and each subsequent number is the sum of the three numbers before it. Thus, tri(0) = 0, tri(1) = 0, tri(2) = 1, tri(n) = well, you know... Define a function that when passed an integer, returns the Tribonacci number at that index. Note: the difficulty is in solving this problem such that you DON'T call the function three times (during each call).