gistfile1.fs

type Person = { Name: string; Flag: bool }

let sortedKeyList = 
    List.concat
    >> List.map (fun { Name = name } -> name)
    >> Seq.distinct
    >> Seq.sort
    >> String.concat ", "
    >> (function "" -> "<none>" | s -> s)

[<Fact>]
let ``Sorted key list``() =
    let persons = List.map (fun name -> { Name = name; Flag = true })
    [
     [],       [],                  "<none>"
     ["fred"], ["barney"; "wilma"], "barney, fred, wilma"
     ["fred"], [],                  "fred"
     ["fred"], ["fred"; "barney"],  "barney, fred"
    ]
    |> List.iter (fun (a, b, res) -> sortedKeyList [a |> persons; b |> persons] =? res)