dancers_partner.fsx

let partner (a : char, b : char) : Move =
    let swap (a, b) = (b, a) in
    let toKV arr =  Map.ofArray (Array.map swap (Array.indexed arr))
    let ofKV map = Array.map snd (Array.sortBy fst (Array.map swap (Map.toArray map))) in
    let withKV f arr = ofKV(f (toKV arr)) in
    withKV (fun kv ->
            let valA = (Map.find a kv) in
            let valB = (Map.find b kv) in
            Map.add b valA (Map.add a valB kv))