external save : t -> Unix.file_descr -> unit = "caml_bp_save"
external load : Unix.file_descr -> t = "caml_bp_load"
external first_child : t -> int -> int = "caml_bp_first_child" "noalloc"
external save : t -> Unix.file_descr -> unit = "caml_bp_save"
external load : Unix.file_descr -> t = "caml_bp_load"
external first_child : t -> int -> int = "caml_bp_first_child" "noalloc"