code: function asyncUnion() {
      return asyncArray(when.apply(null, argsToArray(arguments)).chain(function (result) {
          return union.apply(array, (normalizeResult(result)).map(function (arg) {
              return isArray(arg) ? arg : [arg];
          }));
      }));
  }
docs: Async version of {@link comb.array.union}.
  
  ```
  function asyncArr(){
  var ret = new comb.Promise();
  process.nextTick(ret.callback.bind(ret, [1,2,3,4,5]);
  return ret.promise;
  }
  
  comb.async.array(asyncArr()).union([3],[7], [9,10]).then(function(union){
  console.log(union); //[1,2,3,4,5,7,9,10]
  });
  
  comb.async.union(asyncArr(), [3],[7], [9,10]).then(function(union){
  console.log(union); //[1,2,3,4,5,7,9,10]
  });
  
  ```
  
  @static
  @memberof comb.async
  @name union