See also

[link:lorgonblog.spaces.live.com]

It simply tells unfold to continue with the generative sequence. To stop use "None".

On a broader scale you'll find the Option type (Some x or None) used where you in C# would find null's scattered around.

For example consider the not to uncommon case of reading each line from a TextReader (file or Console.In for example) with unfold you could accomplish that like so

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#light

open System
open System.IO

let input = new StringReader("Hello\r\nWorld\r\nBye.\r\n") :> TextReader

let ReadLine (stream:TextReader) = 
    let line = stream.ReadLine()
    if line <> null then
        Some(line, stream)
    else
        None    

Seq.unfold ReadLine input
|> print_any