get xml() {
          var ret = "";

          // create string containing concatenation of all (but last) Attribute string values (separated by spaces)
          for (var i=0; i < this.length -1; i++) {
            ret += this[i].xml +" ";
          }

          // add last Attribute to string (without trailing space)
          if (this.length > 0) {
            ret += this[this.length -1].xml;
          }

          return ret;
    }
@class  NamedNodeMap -
     used to represent collections of nodes that can be accessed by name
     typically a set of Element attributes

@extends NodeList -
     note W3C spec says that this is not the case, but we need an item()
     method identical to NodeList's, so why not?
@param  ownerDocument : Document - the ownerDocument
@param  parentNode    : Node - the node that the NamedNodeMap is attached to (or null)