function increment (amount) {
      if (sessions > 0) {
        progress.value = inc(progress.value, amount)
      }

      return sessions
    }
We're trying to avoid side effects if start() is called inside a watchEffect()
so we're accessing the _value property directly (under the covers implementation detail of ref())

Otherwise, any refs() accessed here would be marked as deps for the watchEffect()
-- and we are changing them below, which would cause an infinite loop