b.Fatalf("target fails with no changes and all changes")
	}

	// Compute minimum number of bits needed to distinguish
	// all the changes we saw during N and all the changes we saw during Y.
	b.SkipHexDigits = skipHexDigits(runN.MatchIDs, runY.MatchIDs)

	// Loop finding and printing change sets, until none remain.
	found := 0
	for {
		// Find set.
		bad := b.search(broken)
		if bad == nil {
			if found == 0 {