let map1 = Map.ofList [("A", "1"); ("B", "2"); ("C", "3")]
map1 |> printfn "%A"
let rev map: Map<string,string> =
Map.fold (fun m key value -> m.Add(value,key)) Map.empty map
rev map1 |> printfn "%A"
bGV0IG1hcDEgPSBNYXAub2ZMaXN0IFsoIkEiLCAiMSIpOyAoIkIiLCAiMiIpOyAoIkMiLCAiMyIpXQoKbWFwMSB8PiBwcmludGZuICIlQSIKCmxldCByZXYgbWFwOiBNYXA8c3RyaW5nLHN0cmluZz4gPSAKICAgICAgTWFwLmZvbGQgKGZ1biBtIGtleSB2YWx1ZSAtPiBtLkFkZCh2YWx1ZSxrZXkpKSBNYXAuZW1wdHkgbWFwCgpyZXYgbWFwMSB8PiBwcmludGZuICIlQSIK