gistfile1.fs

let sortedKeyList = 
    Seq.map (Map.toSeq >> Seq.map fst)
    >> Seq.concat
    >> Seq.distinct
    >> Seq.sort
    >> String.concat ", "
    >> (function "" -> "<none>" | s -> s)

[<Fact>]
let ``Sorted key list``() =
    [[],             [],                              "<none>"
     ["fred", true], ["barney", true; "wilma", true], "barney, fred, wilma"
     ["fred", true], [],                              "fred"
     ["fred", true], ["fred", true; "barney", true],  "barney, fred"]
    |> List.iter (fun (m1, m2, res) -> sortedKeyList [Map.ofSeq m1; Map.ofSeq m2] =? res)