implemented prefixed (as in block level import) @extend

lib/visitor/normalizer.js

@@ -403,10 +403,13 @@ Normalizer.prototype.extend = function(group, selectors){
     var groups = map[extend.selector];
     if (!groups) {
       if (extend.optional) return;
-      var err = new Error('Failed to @extend "' + extend.selector + '"');
-      err.lineno = extend.lineno;
-      err.column = extend.column;
-      throw err;
+      groups = self._checkForPrefixedGroups(extend.selector);
+      if(!groups) {
+        var err = new Error('Failed to @extend "' + extend.selector + '"');
+        err.lineno = extend.lineno;
+        err.column = extend.column;
+        throw err;
+      }
     }
     selectors.forEach(function(selector){
       var node = new nodes.Selector;
@@ -422,3 +425,17 @@ Normalizer.prototype.extend = function(group, selectors){
 
   group.block = this.visit(group.block);
 };
+
+Normalizer.prototype._checkForPrefixedGroups = function(selector){
+  var prefix = [];
+  var map = this.map;
+  var result = null;
+  this.stack.find(function(stackElementArray){
+    var stackElement = stackElementArray[0];
+      prefix.push(stackElement.val);
+      var fullSelector = prefix.join(" ") + " " + selector;
+      result = map[fullSelector];
+      return result != null;
+  });
+  return result;
+};