nth_tail(n, xs)
Return the nth tail of the the sequence xs,
return false if xs is not a valid sequence.
nth_tail(3, [1, 2, 3, 4, 5])
// [4, 5]
function ints(n) n:~ints(n+1)
let s = ints(100)
nth_tail(20, s)
// 120:<promise>