diff --git a/BUGS.md b/BUGS.md
index 767dfd7..a8e0d26 100644
--- a/BUGS.md
+++ b/BUGS.md
@@ -2,13 +2,10 @@
 BUGS:
 
 - se refs de um topic estao invalidos, nao carrega esse topico. devia haver um error nos logs a indicar qual o ref invalido.
-- templates not working: quesntion-information, question-warning (remove all informative panels??)
-- link directo para topico nao valida se topico esta unlocked.
-- reportar comentarios após submeter.
 - forçar reload das perguntas sem ter de deitar abaixo o servidor.
 - topicos virtuais nao deveriam aparecer. na construção da árvore os sucessores seriam ligados directamente aos predecessores.
 - Criar outra estrutura organizada em capítulos (conjuntos de tópicos). Permitir capítulos de capítulos, etc. talvez usar grafos de grafos...
-- pertuntas tipo tristate: (sim, não, não sei)
+- pertuntas tipo tristate: (sim, não, não sei
 - detect questions in questions.yaml without ref -> error ou generate default.
 - error if demo.yaml has no topics
 - session management. close after inactive time.
@@ -19,6 +16,8 @@ BUGS:
 
 TODO:
 
+- reportar comentarios após submeter.
+- each topic only loads a sample of K questions (max) in random order.
 - servir imagens/ficheiros.
 - update de fontawesome para versão 5.
 - reload das perguntas enquanto online.
@@ -35,6 +34,8 @@ TODO:
 
 FIXED:
 
+- link directo para topico nao valida se topico esta unlocked.
+- templates not working: quesntion-information, question-warning (remove all informative panels??)
 - enderecos errados dao internal error.
 - barra de progresso nao está visível.
 - tabs em textarea nao funcionam correctamente (insere 1 espaco em vez de 4)
diff --git a/knowledge.py b/knowledge.py
index c662210..04b92cc 100644
--- a/knowledge.py
+++ b/knowledge.py
@@ -70,6 +70,10 @@ class StudentKnowledge(object):
         if not topic:
             topic = self.get_recommended_topic()
 
+        # check if it's unlocked
+        if self.is_locked(topic):
+            return False
+
         self.current_topic = topic
         # logger.info(f'Topic set to "{topic}"')
 
@@ -82,6 +86,7 @@ class StudentKnowledge(object):
 
         self.current_question = self.questions.pop(0)  # FIXME crash if empty
         self.current_question['start_time'] = datetime.now()
+        return True
 
     # ------------------------------------------------------------------------
     # The topic has finished and there are no more questions.
@@ -148,6 +153,10 @@ class StudentKnowledge(object):
         return self.current_topic
 
     # ------------------------------------------------------------------------
+    def is_locked(self, topic):
+        return topic not in self.state
+
+    # ------------------------------------------------------------------------
     # Return list of {ref: 'xpto', name: 'long name', leve: 0.5}
     # Levels are in the interval [0, 1] if unlocked or None if locked.
     # Topics unlocked but not yet done have level 0.0.
@@ -176,3 +185,4 @@ class StudentKnowledge(object):
     # ------------------------------------------------------------------------
     def get_recommended_topic(self):    # FIXME untested
         return min(self.state.items(), key=lambda x: x[1]['level'])[0]
+
diff --git a/learnapp.py b/learnapp.py
index 52c444a..f8e9b45 100644
--- a/learnapp.py
+++ b/learnapp.py
@@ -147,12 +147,16 @@ class LearnApp(object):
     # ------------------------------------------------------------------------
     def start_topic(self, uid, topic):
         try:
-            self.online[uid]['state'].init_topic(topic)
+            ok = self.online[uid]['state'].init_topic(topic)
         except KeyError as e:
-            logger.warning(f'User "{uid}" trying to start nonexistent "{topic}"')
+            logger.warning(f'User "{uid}" denied nonexistent "{topic}"')
             raise e
         else:
-            logger.info(f'User "{uid}" started "{topic}"')
+            if ok:
+                logger.info(f'User "{uid}" started "{topic}"')
+            else:
+                logger.warning(f'User "{uid}" denied locked "{topic}"')
+            return ok
 
     # ------------------------------------------------------------------------
     # Fill db table 'Topic' with topics from the graph if not already there.
diff --git a/serve.py b/serve.py
index bb89d79..d768db2 100755
--- a/serve.py
+++ b/serve.py
@@ -129,16 +129,17 @@ class TopicHandler(BaseHandler):
         uid = self.current_user
 
         try:
-            self.learn.start_topic(uid, topic)
+            ok = self.learn.start_topic(uid, topic)
         except KeyError:
-            self.redirect('/')            
-            # raise tornado.web.HTTPError(404)
+            self.redirect('/')
         else:
-            self.render('topic.html',
-                uid=uid,
-                name=self.learn.get_student_name(uid),
-                )
-
+            if ok:
+                self.render('topic.html',
+                    uid=uid,
+                    name=self.learn.get_student_name(uid),
+                    )
+            else:
+                self.redirect('/')
 
 # ----------------------------------------------------------------------------
 class FileHandler(BaseHandler):
--
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