docs: function box() {
    var args = mlutil.asArray.apply(null, arguments);
    switch(args.length) {
    case 0:
      throw new Error('missing four corners for box');
    case 1:
      if (args[0] instanceof BoxDef) {
        return new BoxRegionDef(args[0]);
      }
      throw new Error('invalid box corners: '+mlutil.identify(args[0], true));
    case 2:
      throw new Error('has two corners but missing two corners for box');
    case 3:
      throw new Error('has three corners but missing one corner for box');
    case 4:
      return new BoxRegionDef(new BoxDef(args[0], args[1], args[2], args[3]));
    default:
      throw new Error('too many arguments for box: '+args.length);
    }
  }