Enable to extend selector groups

Code was actually working. Not sure why we had this
check if the first place (was added with first commit
when expand.cpp was introduced in the project). So I
just removed the check with the error and it worked!

Fixes sass#950

expand.cpp

@@ -436,26 +436,29 @@ namespace Sass {
     To_String to_string(&ctx);
     Selector_List* extender = static_cast<Selector_List*>(selector_stack.back());
     if (!extender) return 0;
-    Selector_List* org_extendee = static_cast<Selector_List*>(e->selector());
-    Selector_List* extendee = static_cast<Selector_List*>(org_extendee->perform(contextualize_eval->with(0, env, backtrace)));
-    if (extendee->length() != 1) {
-      error("selector groups may not be extended", extendee->pstate(), backtrace);
-    }
-    Complex_Selector* c = (*extendee)[0];
-    if (!c->head() || c->tail()) {
-      error("nested selectors may not be extended", c->pstate(), backtrace);
-    }
-    Compound_Selector* s = c->head();
-    s->is_optional(org_extendee->is_optional());
-    // // need to convert the compound selector into a by-value data structure
-    // vector<string> target_vec;
-    // for (size_t i = 0, L = s->length(); i < L; ++i)
-    // { target_vec.push_back((*s)[i]->perform(&to_string)); }
-
-    for (size_t i = 0, L = extender->length(); i < L; ++i) {
-      // let's test this out
-      // cerr << "REGISTERING EXTENSION REQUEST: " << (*extender)[i]->perform(&to_string) << " <- " << s->perform(&to_string) << endl;
-      ctx.subset_map.put(s->to_str_vec(), make_pair((*extender)[i], s));
+    Contextualize_Eval* eval = contextualize_eval->with(0, env, backtrace);
+    Selector_List* selector_list = static_cast<Selector_List*>(e->selector());
+    Selector_List* contextualized = static_cast<Selector_List*>(selector_list->perform(eval));
+    // ToDo: remove once feature proves stable!
+    // if (contextualized->length() != 1) {
+    //   error("selector groups may not be extended", extendee->pstate(), backtrace);
+    // }
+    for (auto complex_sel : contextualized->elements()) {
+      Complex_Selector* c = complex_sel;
+      if (!c->head() || c->tail()) {
+        error("nested selectors may not be extended", c->pstate(), backtrace);
+      }
+      Compound_Selector* compound_sel = c->head();
+      compound_sel->is_optional(selector_list->is_optional());
+      // // need to convert the compound selector into a by-value data structure
+      // vector<string> target_vec;
+      // for (size_t i = 0, L = compound_sel->length(); i < L; ++i)
+      // { target_vec.push_back((*compound_sel)[i]->perform(&to_string)); }
+      for (size_t i = 0, L = extender->length(); i < L; ++i) {
+        // let's test this out
+        // cerr << "REGISTERING EXTENSION REQUEST: " << (*extender)[i]->perform(&to_string) << " <- " << compound_sel->perform(&to_string) << endl;
+        ctx.subset_map.put(compound_sel->to_str_vec(), make_pair(extender[i], compound_sel));
+      }
     }
     return 0;
   }