code: function(tag, nodes) {
      var xmlNodes = nodes.map(function(x) {return x.xml(xml, opt_brief);});
      var tagNode = xml.createElementNS('', tag);
      for (var i = 0, child; child = xmlNodes[i]; i++) {
        tagNode.appendChild(child);
      }
      return tagNode;
    }
docs: Translates a list of nodes into XML representation.
  @param {string} tag Name of the enclosing tag.
  @param {!Array.<!sre.SemanticNode>} nodes A list of nodes.
  @return {Node} An XML representation of the node list.