Preserve append history across tiers

internal/engine/compose.go

@@ -16,6 +16,20 @@ type composedDocument struct {
 	inputNodes     int
 }
 
+type sequenceHistoryMode uint8
+
+const (
+	sequenceHistoryReplace sequenceHistoryMode = iota + 1
+	sequenceHistoryAppend
+)
+
+type sequenceHistoryContribution struct {
+	path   Path
+	mode   sequenceHistoryMode
+	offset int
+	length int
+}
+
 // assemblyAccumulator owns one mutable working document. Acquired trees held by
 // history applications are separate from its root and remain unchanged.
 type assemblyAccumulator struct {
@@ -113,8 +127,24 @@ func composeDocumentsWithPolicy(policy sequencePolicy, inputs ...*document.Node)
 		mergeComposedMappings(root, input, Path{}, index, sourceIndex+1, policy, validated)
 	}
 	applications := make([]historyApplication, 0, len(inputs))
-	for _, input := range inputs {
-		applications = append(applications, historyApplication{layer: LayerBase, value: input})
+	seenSequences := make(map[string]bool)
+	for sourceIndex, input := range inputs {
+		application := historyApplication{layer: LayerBase, value: input}
+		for _, assigned := range ranges {
+			if assigned.sourceIndex != sourceIndex {
+				continue
+			}
+			pointer := assigned.path.String()
+			mode := sequenceHistoryAppend
+			if !seenSequences[pointer] {
+				mode = sequenceHistoryReplace
+			}
+			seenSequences[pointer] = true
+			application.sequences = append(application.sequences, sequenceHistoryContribution{
+				path: assigned.path.clone(), mode: mode, offset: assigned.start, length: assigned.end - assigned.start,
+			})
+		}
+		applications = append(applications, application)
 	}
 	origins := projectedCompositionOrigins(inputs, ranges)
 	completeCompositionOrigins(root, Path{}, origins)