let max = 1000
var a = Array.Empty(max, 0)
for n in 0..(max-2) {
    var m = n - 1
    while m >= 0 {
        if a[m] == a[n] {
            a[n+1] = n - m
            break
        }
        m -= 1
    }
}
print("The first ten terms of the Van Eck sequence are: \(a[0..10].ToArray())")
print("Terms 991 to 1000 of the sequence are: \(a[991..999].ToArray())")