Define a function, trinum, that returns a stream ("infinite list") representing the triangular numbers, which is the sequence 1,3,6,10,15,21,28,.... Note that the difference between numbers increases by one, i.e., 3 = 1+2, 6 = 3 + 3, so in general, tn+1 = tn + n+1 (starting with t = 1). As a reminder, here is our definition for the stream datatype: datatype 'a stream Nil | Cons of 'a (unit> 'a stream); Here is a sample execution, using our printStrm function: val f trinum(); val f - Cons (1, fn): int stream printStrm 10 f; - 1 3 6 10 15 21 M 28 36 45 55 You don't need to know about the implementation of printStrm, nor any other stream functions: you just need to know about Cons and fn in your definition of trinum). You will want a nested function definition inside of trinum, like you did with fib in the homework.