function _concat(set1, set2) {
    set1 = set1 || [];
    set2 = set2 || [];
    var idx;
    var len1 = set1.length;
    var len2 = set2.length;
    var result = [];
    idx = 0;
    while (idx < len1) {
      result[result.length] = set1[idx];
      idx += 1;
    }
    idx = 0;
    while (idx < len2) {
      result[result.length] = set2[idx];
      idx += 1;
    }
    return result;
  }
Private `concat` function to merge two array-like objects.

@private
@param {Array|Arguments} [set1=[]] An array-like object.
@param {Array|Arguments} [set2=[]] An array-like object.
@return {Array} A new, merged array.
@example

     _concat([4, 5, 6], [1, 2, 3]); //=> [4, 5, 6, 1, 2, 3]