nth(n, s)
Return the element at index n of the sequence s. If s is not a valid sequence or if n
is not a valid index, return false.
nth(3, [1, 2, 3, 4, 5])
// 4
function ints(n) n:~ints(n + 2) // an infinite sequence of integers
let s = ints(10)
nth(0, s)
// 10
nth(100, s)
// 210