Close cross-cutting sequence regression gaps

internal/engine/history_sequence_test.go

@@ -17,6 +17,14 @@ func TestSequenceHistoryReplaysAppendAtAssignedIndexes(t *testing.T) {
 	if len(diagnostics) != 0 {
 		t.Fatalf("compose diagnostics = %#v", diagnostics)
 	}
+	for applicationIndex, application := range composed.applications {
+		for path, want := range map[string]int{"/records": 1, "/other": 1} {
+			sequence, present := documentAtPath(application.value, mustParsePath(t, path))
+			if !present || sequence.Kind != document.Sequence || len(sequence.Sequence) != want {
+				t.Errorf("application %d retained %s = %#v, %v; want one source fragment item", applicationIndex, path, sequence, present)
+			}
+		}
+	}
 
 	working := parseCompositionDocument(t, "defaults", "records: [{id: default}]\nother: [{id: default}]\n")
 	index := new(mappingIndex)