type 'a starred_level = Root | Starring_Left of 'a starred_nonroot | Starring_Right of 'a starred_nonroot
and 'a starred_nonroot = { parent : 'a starred_level; sibling: 'a tree };;
type 'a starred_level = Root | Starring_Left of 'a starred_nonroot | Starring_Right of 'a starred_nonroot
and 'a starred_nonroot = { parent : 'a starred_level; sibling: 'a tree };;