docs: function boost() {
    var args = mlutil.asArray.apply(null, arguments);
    switch(args.length) {
    case 0:
      throw new Error('missing matching and boosting queries');
    case 1:
      throw new Error('has matching but missing boosting query: '+mlutil.identify(args[0], true));
    case 2:
      return new BoostDef(new MatchingBoostingDef(args[0], args[1]));
    default:
      throw new Error('too many arguments for boost(): '+args.length);
    }
  }