Is there a stable sort that works with partial orders in the standard library?

Question

I have lists of tasks that I want to sort pseudo-topologically. Say, I have something like this:

enum Task {
    case a(Int, Int)
    case b(String)
    case c(Bool)
}

I require all a tasks to be executed before any b tasks, and all b tasks before all c tasks. In addition, all tasks of the same time need to remain in the same order as in the input.

In computer science lingo, what I need is

• a stable sorting algorithm and
• a way to feed it a partial order.

Is there such a thing in the Swift standard library?

Observations:

• Comparable, a natural protocol to look at for sorting, requires the order to be total.
• sorted can be made to work with partial orders, but is not stable.

As an alternative, a topological sorting algorithm would work as well, even though it would do more than I need.