Shared via Pony Playground (https://playground.ponylang.org/?gist=679bacf1177db8df7a8a6a2f273e5b56)

main.pony

actor Main
  new create(env: Env) =>
    None

primitive TokenCount
primitive TokenItem

type Token[A: Any #share] is ((TokenCount, None, USize) | (TokenItem, A))

interface Tokens[A: Any #share]
  fun next_count(iter: Iterator[Token[A]]): USize? =>
    match iter.next()?
    | (let _: TokenCount, let _: None, let count: USize) => count
    else error
    end
  
  fun next_item(iter: Iterator[Token[A]]): A? =>
    match iter.next()?
    | (let _: TokenItem, let item: A) => item
    else error
    end