code: function(cb){
  			if (!fnRetrieveMessages)
  				return cb();
  			console.log("will retrieve messages");
  			fnRetrieveMessages(conn, last_ball_mci, bMultiAuthored, arrPayingAddresses, function(err, arrMoreMessages, assocMorePrivatePayloads){
  				console.log("fnRetrieveMessages callback: err code = "+(err ? err.error_code : ""));
  				if (err)
  					return cb((typeof err === "string") ? ("unable to add additional messages: "+err) : err);
  				Array.prototype.push.apply(objUnit.messages, arrMoreMessages);
  				if (assocMorePrivatePayloads && Object.keys(assocMorePrivatePayloads).length > 0)
  					for (var payload_hash in assocMorePrivatePayloads)
  						assocPrivatePayloads[payload_hash] = assocMorePrivatePayloads[payload_hash];
  				cb();
  			});
  		}
docs: messages retrieved via callback