Trace_util.Multi_collectorCombine multiple collectors into one
val combine_l : Trace_core.Collector.t list -> Trace_core.Collector.tCombine multiple collectors, ie return a collector that forwards to every collector in the list.
val combine :
Trace_core.Collector.t ->
Trace_core.Collector.t ->
Trace_core.Collector.tcombine s1 s2 is a collector that forwards every call to s1 and s2 both.