How do I sort a collection of Lists in lexicographic order in Scala?

Inspired by Ben Lings' answer, I managed to work out what seems like the simplest way to sort lists lexicographically: add the line

import scala.math.Ordering.Implicits._

before doing your List[Int] comparison, to ensure that the implicit function infixOrderingOps is in scope.

(11 minutes ago I actually didn't know how to do this, I hope it's considered okay to answer my own question.)

implicit def List2OrderedList[A <% Ordered[A]](list1: List[A]): Ordered[List[A]] = { 
    new Ordered[List[A]] {
        def compare(list2: List[A]): Int = {
            for((x,y) <- list1 zip list2) {
                val c = x compare y
                if(c != 0) return c
            }
            return list1.size - list2.size
        }
    }
}

An important thing to note here is the 'view bound' A <% Ordered[A], which ensures that A needn't itself by an Ordered[A], just that there's a way to do this conversion. Happily, the Scala library's object Predef has an implicit conversion from Ints to RichInts, which in particular are Ordered[Int]s.

The rest of the code is just implementing lexicographic ordering.