You're assuming that x+[5] mutates the list instead of returning a new list. If you're planning to have multiple threads append items to a list, the immutable

You're assuming that x+[5] mutates the list instead of returning a new list. If you're planning to have multiple threads append items to a list, the immutable way to do it is to have each thread return a list of items to append to the main list, then fold those items into the list.