export default function(onenter, onupdate, onexit) {
  var enter = this.enter(), update = this, exit = this.exit();
  if (typeof onenter === "function") {
    enter = onenter(enter);
    if (enter) enter = enter.selection();
  } else {
    enter = enter.append(onenter + "");
  }
  if (onupdate != null) {
    update = onupdate(update);
    if (update) update = update.selection();
  }
  if (onexit == null) exit.remove(); else onexit(exit);
  return enter && update ? enter.merge(update).order() : update;
}