How to use exclusivity with @Flag?

I'm trying to figure out how to use the @Flag's exclusivity property.

I have the following:

struct CharacterCount: ParsableCommand {
  enum CharSet: String, CaseIterable {
    case whitespace
    case numbers
    case vowels
  @Argument(help: "String to count the characters of") var string: String
  @Flag(default: CharSet.whitespace, exclusivity: .exclusive, help: "") var characterSets: CharSet
  func run() throws {

What I'd like to do is to be able to pass in something like this to my tool:

CharacterCount Alice --whitespace --number

And have the tool only grab one. I know I can do this with exclusivity, but I can't figure out how to get it work. If I make characterSets an array, it doesn't compile anymore:

Cannot convert value of type 'Flag<CharacterCount.CharSet>' to specified type '[CharacterCount.CharSet]'

If I don't mark it as an array and pass more than one case, the tool crashes with:

Error: Value at position InputOrigin(_elements: Set([ArgumentParser.InputOrigin.Element.argumentIndex(2)])) has already been set from value at position InputOrigin(_elements: Set([ArgumentParser.InputOrigin.Element.argumentIndex(1)])).
Usage: character-count <string> [--whitespace] [--numbers] [--vowels]

I wasn't able to find the answer to this in the docs.

Aside from .exclusive, there are .chooseFirst, and .chooseLast which might be what you want.