code: function requestUnfinishedPastUnitsOfPrivateChains(arrChains, onDone){
  	mutex.lock(["private_chains"], function(unlock){
  		function finish(){
  			unlock();
  			if (onDone)
  				onDone();
  		}
  		privatePayment.findUnfinishedPastUnitsOfPrivateChains(arrChains, true, function(arrUnits){
  			if (arrUnits.length === 0)
  				return finish();
  			breadcrumbs.add(arrUnits.length+" unfinished past units of private chains");
  			requestHistoryFor(arrUnits, [], finish);
  		});
  	});
  }
docs: light only