function _getDocumentIdentifier(doc) {
    return doc._wysihtml_identifier || (doc._wysihtml_identifier = DOCUMENT_IDENTIFIER++);
  }
High performant way to check whether an element with a specific class name is in the given document
Optimized for being heavily executed
Unleashes the power of live node lists

@param {Object} doc The document object of the context where to check
@param {String} tagName Upper cased tag name
@example
   wysihtml.dom.hasElementWithClassName(document, "foobar");