func rangeFormat(a) {
    if a.Length() == 0 {
        return ""
    }
    var parts = []
    var n1 = 0
    while true {
        var n2 = n1 + 1
        while n2 < a.Length() && a[n2] == a[n2-1]+1 {
            n2 += 1
        }
        var s = a[n1].ToString()
        if n2 == n1+2 {
            s += "," + a[n2-1]
        } else if n2 > n1+2 {
            s += "-" + a[n2-1]
        }
        parts.Add(s)
        if n2 == a.Length() {
            break
        }
        if a[n2] == a[n2-1] {
            throw "Sequence repeats value \(a[n2])"
        }
        if a[n2] < a[n2-1] {
            throw "Sequence not ordered: \(a[n2]) < \(a[n2-1])"
        }
        n1 = n2
    }
    return String.Join(values: parts)
}

var rf = rangeFormat([
    0, 1, 2, 4, 6, 7, 8, 11, 12, 14,
    15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
    25, 27, 28, 29, 30, 31, 32, 33, 35, 36,
    37, 38, 39
])
print("range format: \(rf)")