external next_sibling : t -> int -> int = "caml_bp_next_sibling" "noalloc"
external preorder_rank : t -> int -> int = "caml_bp_preorder_rank" "noalloc"
external next_sibling : t -> int -> int = "caml_bp_next_sibling" "noalloc"
external preorder_rank : t -> int -> int = "caml_bp_preorder_rank" "noalloc"