public static IHierarchyObject GetCommonAncestor(this IHierarchyObject source, IHierarchyObject other)
        {
            if (object.ReferenceEquals(other, source)) {
                return source;

            } else if (other.IsAncestorOf(source)) {
                return other;

            } else if (other.IsDescendentOf(source)) {
                return source;

            } else {
                // Scan the hierarchy
                foreach (IHierarchyObject ancestor in GetAncestors(other)) {
                    // Check the ancestry - skip the starting instance; we already
                    // performed that check
                    if (ancestor.ParentObject != null
                        && ancestor.ParentObject.IsAncestorOf(source)) {

                        return ancestor.ParentObject;
                    }
                }

                return null;

            }
        }