val b_object: 'expr -> 'expr box
val b_indent: 'expr box -> 'expr box
val b_space: 'expr box
-val b_kw: string -> 'expr box
+(* val b_kw: string -> 'expr box *)
val b_toggle: 'expr box list -> 'expr box (** action which toggle among items *)
val pp_attr: attr -> string