-val trace : int -> int -> ('a, Format.formatter, unit, unit) format4 -> 'a
+val trace : ?msg:string -> int -> int -> StateSet.t -> StateSet.t -> unit
+(** [trace nid round d t msg] records the message [msg] together
+ with the a node preorder [nid], the [round], the [d]one set and
+ the [t]odo set *)