if (existing != null) return existing

    if (computed == null) {
      computed = compute()
    }

    // If we successfully add the computed element, we're done.
    val newTags = tags.plus(type, computed)
    if (compareAndSet(tags, newTags)) return computed

    // We lost the race. Possibly to other code that was putting a *different* key. Try again!
  }