code: function asyncSum(arr) {
      return when(arr).chain(function (result) {
          return sum.call(array, normalizeResult(result));
      });
  }
docs: Async version of {@link comb.array.sum}.
  
  ```
  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()).sum().then(function(sum){
  console.log(sum) //15
  })
  
  comb.async.sum(asyncArr()).then(function(sum){
  console.log(sum) //15
  })
  
  ```
  
  @static
  @memberof comb.async
  @name sum