code: function(el, tagName, tagData) {
                  var helperTagCallback = tagData.options.attr('tags.' + tagName),
                      tagCallback = helperTagCallback || tags[tagName];
  
                  // If this was an element like <foo-bar> that doesn't have a component, just render its content
                  var scope = tagData.scope,
                      res;
  
                  if (tagCallback) {
                      var reads = can.__clearReading();
                      res = tagCallback(el, tagData);
                      can.__setReading(reads);
                  } else {
                      res = scope;
                  }
  
  
  
                  // If the tagCallback gave us something to render with, and there is content within that element
                  // render it!
                  if (res && tagData.subtemplate) {
  
                      if (scope !== res) {
                          scope = scope.add(res);
                      }
                      var result = tagData.subtemplate(scope, tagData.options);
                      var frag = typeof result === "string" ? can.view.frag(result) : result;
                      can.appendChild(el, frag);
                  }
              }
docs: handles calling back a tag callback