},
 
     clone: function() {
-        var clone = this._$.clone(true, true);
+        var clone = this._$.clone(true, true),
+            node = this;
+        clone.find('*').addBack().each(function() {
+            var el = this,
+                clonedData = $(this).data();
+
+            _.pairs(clonedData).forEach(function(pair) {
+                var key = pair[0],
+                    value = pair[1];
+                if(_.isFunction(value.clone)) {
+                    clonedData[key] = value.clone(node.document.createDocumentNode(el));
+                }
+            });
+        });
         return this.document.createDocumentNode(clone[0]);
     },
 
 
 
 var parseXML = function(xml) {
-    return $($.trim(xml))[0];
+    var toret = $($.trim(xml));
+    if(!toret.length) {
+        throw new Error('Unable to parse XML: ' + xml);
+    }
+    return toret[0];
+
 };
 
 var registerTransformation = function(desc, name, target) {
 };
 
 
-var Document = function(xml) {
-    this.loadXML(xml);
+var Document = function(xml, extensions) {
     this.undoStack = [];
     this.redoStack = [];
     this._transactionStack = [];
     this._elementNodeTransformations = {};
     
     this.registerExtension(coreTransformations);
+
+    (extensions || []).forEach(function(extension) {
+        this.registerExtension(extension);
+    }.bind(this));
+    this.loadXML(xml);
 };
 
 $.extend(Document.prototype, Backbone.Events, {
                     /* globals document */
                     from = document.createTextNode(from.text);
                 } else {
+                    if(!from.tagName) {
+                        throw new Error('tagName missing');
+                    }
                     var node = $('<' + from.tagName + '>');
 
                     _.keys(from.attrs || {}).forEach(function(key) {