Activate selected overlay collection policies

internal/engine/recursive_merge.go

@@ -9,6 +9,7 @@ type recursiveMergeHooks struct {
 	added    func(Path, *document.Node)
 	combined func(Path, *document.Node)
 	replaced func(Path, *document.Node, *document.Node)
+	merge    func(Path, *document.Node, *document.Node) (*document.Node, bool)
 }
 
 // mergeRecursiveMappings changes an owned mapping and never aliases incoming
@@ -18,6 +19,14 @@ func mergeRecursiveMappings(target, incoming *document.Node, current Path, index
 		path := current.Key(entry.Key)
 		position := index.find(target, entry.Key)
 		if position < 0 {
+			if hooks.merge != nil {
+				if merged, handled := hooks.merge(path, nil, entry.Value); handled {
+					cloned := entry
+					cloned.Value = document.Clone(merged)
+					index.append(target, cloned)
+					continue
+				}
+			}
 			cloned := entry
 			cloned.Value = document.Clone(entry.Value)
 			index.append(target, cloned)
@@ -35,6 +44,13 @@ func mergeRecursiveMappings(target, incoming *document.Node, current Path, index
 			mergeRecursiveMappings(existing, entry.Value, path, index, hooks)
 			continue
 		}
+		if hooks.merge != nil {
+			if merged, handled := hooks.merge(path, existing, entry.Value); handled {
+				target.Mapping[position] = entry
+				target.Mapping[position].Value = document.Clone(merged)
+				continue
+			}
+		}
 
 		if hooks.replaced != nil {
 			hooks.replaced(path, existing, entry.Value)