diff --git a/BUGS.md b/BUGS.md
index bd9adcd..3da7246 100644
--- a/BUGS.md
+++ b/BUGS.md
@@ -1,27 +1,23 @@
 
 # BUGS
 
+- apos clicar no botao responder, inactivar o input (importante quando o tempo de correcção é grande)
 - nao esta a seguir o max_tries definido no ficheiro de dependencias.
-- registar last_seen e remover os antigos de cada vez que houver um login.
 - initdb da integrity error se no mesmo comando existirem alunos repetidos (p.ex em ficheiros csv diferentes ou entre csv e opcao -a)
 - double click submits twice.
-- marking all options right in a radio question breaks!
 - duplo clicks no botao "responder" dessincroniza as questões, ver debounce em https://stackoverflow.com/questions/20281546/how-to-prevent-calling-of-en-event-handler-twice-on-fast-clicks
-- quando termina topico devia apagar as perguntas todas (se falhar a gerar novo topico, aparecem perguntas do antigo)
-- apos clicar no botao responder, inactivar o input (importante quando o tempo de correcção é grande)
 - devia mostrar timeout para o aluno saber a razao.
 - permitir configuracao para escolher entre static files locais ou remotos
 - templates question-*.html tem input hidden question_ref que não é usado. remover?
-- click numa opcao checkbox fora da checkbox+label não está a funcionar.
 - shift-enter não está a funcionar
 - default prefix should be obtained from each course (yaml conf)?
-- tabelas nas perguntas radio/checkbox não ocupam todo o espaço como em question.
 
 # TODO
 
+- checkbox devia ter correct no intervalo [0,1] tal como radio. em caso de desconto a correccção faz 2*x-1. isto permite a mesma semantica nos dois tipos de perguntas.
+- registar last_seen e remover os antigos de cada vez que houver um login.
 - indicar qtos topicos faltam (>=50%) para terminar o curso.
-- use run_script_async to run run_script using asyncio.run?
-- ao fim de 3 tentativas de login, envial email para aluno com link para definir nova password (com timeout de 5 minutos).
+- ao fim de 3 tentativas com password errada, envia email com nova password.
 - mostrar capitulos e subtopicos de forma hierarquica. clicar no capitulo expande as dependencias.
 - mostrar rankings alunos/perguntas respondidas/% correctas/valor esperado topicos.
 - botão não sei...
@@ -33,14 +29,16 @@
 - tabela com perguntas / quantidade de respostas certas/erradas.
 - tabela com topicos / quantidade de estrelas.
 - pymips: activar/desactivar instruções
-- implementar servidor http com redirect para https.
-- ao fim de 3 tentativas com password errada, envia email com nova password.
 - titulos das perguntas não suportam markdown.
 - pagina report que permita ver tabela alunos/topicos, estatisticas perguntas mais falhadas, tempo médio por pergunta.
 - normalizar com perguntations.
 
 # FIXED
 
+- marking all options right in a radio question breaks!
+- implementar servidor http com redirect para https.
+- tabelas nas perguntas radio/checkbox não ocupam todo o espaço como em question.
+- click numa opcao checkbox fora da checkbox+label não está a funcionar.
 - mathjax, formulas $$f(x)$$ nas opções de escolha multipla, não ficam centradas em toda a coluna mas apenas na largura do parágrafo.
 - QFactory.generate() devia fazer run da gen_async, ou remover.
 - classificacoes so devia mostrar os que ja fizeram alguma coisa
diff --git a/aprendizations/learnapp.py b/aprendizations/learnapp.py
index 7ae92ce..9d410d3 100644
--- a/aprendizations/learnapp.py
+++ b/aprendizations/learnapp.py
@@ -16,7 +16,7 @@ import sqlalchemy as sa
 
 # this project
 from .models import Student, Answer, Topic, StudentTopic
-from .questions import Question, QFactory, QDict
+from .questions import Question, QFactory, QDict, QuestionException
 from .student import StudentState
 from .tools import load_yaml
 
@@ -100,7 +100,6 @@ class LearnApp(object):
         # if graph has topics that are not in the database, add them
         self.add_missing_topics(self.deps.nodes())
 
-
         if check:
             self.sanity_check_questions()
 
@@ -113,8 +112,8 @@ class LearnApp(object):
             logger.debug(f'checking {qref}...')
             try:
                 q = self.factory[qref].generate()
-            except Exception:
-                logger.error(f'Failed to generate "{qref}".')
+            except QuestionException as e:
+                logger.error(e)
                 errors += 1
                 continue  # to next question
 
@@ -137,7 +136,7 @@ class LearnApp(object):
                         continue  # to next test
 
         if errors > 0:
-            logger.error(f'{errors:>6} errors found.')
+            logger.error(f'{errors:>6} error(s) found.')
             raise LearnException('Sanity checks')
         else:
             logger.info('     0 errors found.')
@@ -345,7 +344,7 @@ class LearnApp(object):
             logger.info(f'{m:6} topics')
             logger.info(f'{q:6} answers')
 
-    # ============================================================================
+    # ========================================================================
     # Populates a digraph.
     #
     # Nodes are the topic references e.g. 'my/topic'
@@ -400,7 +399,7 @@ class LearnApp(object):
     def make_factory(self) -> Dict[str, QFactory]:
 
         logger.info('Building questions factory:')
-        factory: Dict[str, QFactory] = {}
+        factory = {}
         g = self.deps
         for tref in g.nodes():
             t = g.nodes[tref]
@@ -418,7 +417,7 @@ class LearnApp(object):
             # within the file
             for i, q in enumerate(questions):
                 qref = q.get('ref', str(i))  # ref or number
-                q['ref'] = tref + ':' + qref
+                q['ref'] = f'{tref}:{qref}'
                 q['path'] = topicpath
                 q.setdefault('append_wrong', t['append_wrong'])
 
diff --git a/aprendizations/questions.py b/aprendizations/questions.py
index 82bc440..689f913 100644
--- a/aprendizations/questions.py
+++ b/aprendizations/questions.py
@@ -72,7 +72,6 @@ class QuestionRadio(Question):
     '''
 
     # ------------------------------------------------------------------------
-    # FIXME marking all options right breaks
     def __init__(self, q: QDict) -> None:
         super().__init__(q)
 
@@ -86,18 +85,46 @@ class QuestionRadio(Question):
             'max_tries': (n + 3) // 4  # 1 try for each 4 options
             }))
 
-        # convert int to list, e.g.  correct: 2 --> correct: [0,0,1,0,0]
-        # correctness levels from 0.0 to 1.0 (no discount here!)
+        # check correct bounds and convert int to list,
+        # e.g.  correct: 2 --> correct: [0,0,1,0,0]
         if isinstance(self['correct'], int):
+            if not (0 <= self['correct'] < n):
+                msg = (f'Correct option not in range 0..{n-1} in '
+                       f'"{self["ref"]}"')
+                raise QuestionException(msg)
+
             self['correct'] = [1.0 if x == self['correct'] else 0.0
                                for x in range(n)]
 
-        if len(self['correct']) != n:
-            msg = ('Number of options and correct differ in '
-                   f'"{self["ref"]}", file "{self["filename"]}".')
-            logger.error(msg)
-            raise QuestionException(msg)
-
+        elif isinstance(self['correct'], list):
+            # must match number of options
+            if len(self['correct']) != n:
+                msg = (f'Incompatible sizes: {n} options vs '
+                       f'{len(self["correct"])} correct in "{self["ref"]}"')
+                raise QuestionException(msg)
+            # make sure is a list of floats
+            try:
+                self['correct'] = [float(x) for x in self['correct']]
+            except (ValueError, TypeError):
+                msg = (f'Correct list must contain numbers [0.0, 1.0] or '
+                       f'booleans in "{self["ref"]}"')
+                raise QuestionException(msg)
+
+            # check grade boundaries
+            if self['discount'] and not all(0.0 <= x <= 1.0
+                                            for x in self['correct']):
+                msg = (f'If discount=true, correct values must be in the '
+                       f'interval [0.0, 1.0] in "{self["ref"]}"')
+                raise QuestionException(msg)
+
+            # at least one correct option
+            if all(x < 1.0 for x in self['correct']):
+                msg = (f'At least one correct option is required in '
+                       f'"{self["ref"]}"')
+                raise QuestionException(msg)
+
+        # If shuffle==false, all options are shown as defined
+        # otherwise, select 1 correct and choose a few wrong ones
         if self['shuffle']:
             # lists with indices of right and wrong options
             right = [i for i in range(n) if self['correct'][i] >= 1]
@@ -123,7 +150,7 @@ class QuestionRadio(Question):
             # final shuffle of the options
             perm = random.sample(range(self['choose']), k=self['choose'])
             self['options'] = [str(options[i]) for i in perm]
-            self['correct'] = [float(correct[i]) for i in perm]
+            self['correct'] = [correct[i] for i in perm]
 
     # ------------------------------------------------------------------------
     # can assign negative grades for wrong answers
@@ -131,10 +158,13 @@ class QuestionRadio(Question):
         super().correct()
 
         if self['answer'] is not None:
-            x = self['correct'][int(self['answer'])]
-            if self['discount']:
-                n = len(self['options'])  # number of options
-                x_aver = sum(self['correct']) / n
+            x = self['correct'][int(self['answer'])]  # get grade of the answer
+            n = len(self['options'])
+            x_aver = sum(self['correct']) / n  # expected value of grade
+
+            # note: there are no numerical errors when summing 1.0s so the
+            #       x_aver can be exactly 1.0 if all options are right
+            if self['discount'] and x_aver != 1.0:
                 x = (x - x_aver) / (1.0 - x_aver)
             self['grade'] = x
 
@@ -168,12 +198,32 @@ class QuestionCheckbox(Question):
             'max_tries': max(1, min(n - 1, 3))
             }))
 
+        # must be a list of numbers
+        if not isinstance(self['correct'], list):
+            msg = 'Correct must be a list of numbers or booleans'
+            raise QuestionException(msg)
+
+        # must match number of options
         if len(self['correct']) != n:
-            msg = (f'Options and correct size mismatch in '
-                   f'"{self["ref"]}", file "{self["filename"]}".')
-            logger.error(msg)
+            msg = (f'Incompatible sizes: {n} options vs '
+                   f'{len(self["correct"])} correct in "{self["ref"]}"')
             raise QuestionException(msg)
 
+        # make sure is a list of floats
+        try:
+            self['correct'] = [float(x) for x in self['correct']]
+        except (ValueError, TypeError):
+            msg = (f'Correct list must contain numbers or '
+                   f'booleans in "{self["ref"]}"')
+            raise QuestionException(msg)
+
+        # check grade boundaries  (FUTURE)
+        # if self['discount'] and not all(0.0 <= x <= 1.0
+        #                                 for x in self['correct']):
+        #     msg = (f'If discount=true, correct values must be in the '
+        #            f'interval [0.0, 1.0] in "{self["ref"]}"')
+        #     raise QuestionException(msg)
+
         # if an option is a list of (right, wrong), pick one
         options = []
         correct = []
@@ -526,17 +576,21 @@ class QFactory(object):
                                          stdin=q['stdin'])
             q.update(out)
 
-        # Finally we create an instance of Question()
+        # Get class for this question type
         try:
-            qinstance = self._types[q['type']](QDict(q))  # of matching class
-        except QuestionException as e:
-            logger.error(e)
-            raise e
+            qclass = self._types[q['type']]
         except KeyError:
             logger.error(f'Invalid type "{q["type"]}" in "{q["ref"]}"')
             raise
-        else:
-            return qinstance
+
+        # Finally create an instance of Question()
+        try:
+            qinstance = qclass(QDict(q))
+        except QuestionException as e:
+            # logger.error(e)
+            raise e
+
+        return qinstance
 
     # ------------------------------------------------------------------------
     def generate(self) -> Question:
diff --git a/aprendizations/serve.py b/aprendizations/serve.py
index 920718e..160d9ff 100644
--- a/aprendizations/serve.py
+++ b/aprendizations/serve.py
@@ -254,7 +254,6 @@ class TopicHandler(BaseHandler):
 class FileHandler(BaseHandler):
     @tornado.web.authenticated
     async def get(self, filename):
-        # logger.debug(f'[FileHandler] {filename}')
         uid = self.current_user
         public_dir = self.learn.get_current_public_dir(uid)
         filepath = path.expanduser(path.join(public_dir, filename))
@@ -331,7 +330,7 @@ class QuestionHandler(BaseHandler):
         user = self.current_user
         answer = self.get_body_arguments('answer')  # list
         qid = self.get_body_arguments('qid')[0]
-        logger.debug(f'[QuestionHandler] user={user}, answer={answer}')
+        logger.debug(f'[QuestionHandler] answer={answer}')
 
         # --- check if browser opened different questions simultaneously
         if qid != self.learn.get_current_question_id(user):
diff --git a/config/logger-debug.yaml b/config/logger-debug.yaml
index 45d6b09..96053d5 100644
--- a/config/logger-debug.yaml
+++ b/config/logger-debug.yaml
@@ -2,50 +2,52 @@
 version: 1
 
 formatters:
-    void:
-        format: ''
-    standard:
-        format: '%(asctime)s | %(thread)-15d | %(levelname)-8s | %(module)-10s | %(funcName)-20s | %(message)s'
-        # datefmt: '%H:%M:%S'
+  void:
+    format: ''
+  standard:
+    format: '%(asctime)s | %(levelname)-8s | %(module)-10s | %(funcName)-22s |
+      %(message)s'
+    # | %(thread)-15d
+    # datefmt: '%H:%M:%S'
 
 handlers:
-    default:
-        level: 'DEBUG'
-        class: 'logging.StreamHandler'
-        formatter: 'standard'
-        stream: 'ext://sys.stdout'
+  default:
+    level: 'DEBUG'
+    class: 'logging.StreamHandler'
+    formatter: 'standard'
+    stream: 'ext://sys.stdout'
 
 loggers:
-    '':
-        handlers: ['default']
-        level: 'DEBUG'
-
-    'aprendizations.factory':
-        handlers: ['default']
-        level: 'DEBUG'
-        propagate: false
-
-    'aprendizations.student':
-        handlers: ['default']
-        level: 'DEBUG'
-        propagate: false
-
-    'aprendizations.learnapp':
-        handlers: ['default']
-        level: 'DEBUG'
-        propagate: false
-
-    'aprendizations.questions':
-        handlers: ['default']
-        level: 'DEBUG'
-        propagate: false
-
-    'aprendizations.tools':
-        handlers: ['default']
-        level: 'DEBUG'
-        propagate: false
-
-    'aprendizations.serve':
-        handlers: ['default']
-        level: 'DEBUG'
-        propagate: false
+  '':
+    handlers: ['default']
+    level: 'DEBUG'
+
+  'aprendizations.factory':
+    handlers: ['default']
+    level: 'DEBUG'
+    propagate: false
+
+  'aprendizations.student':
+    handlers: ['default']
+    level: 'DEBUG'
+    propagate: false
+
+  'aprendizations.learnapp':
+    handlers: ['default']
+    level: 'DEBUG'
+    propagate: false
+
+  'aprendizations.questions':
+    handlers: ['default']
+    level: 'DEBUG'
+    propagate: false
+
+  'aprendizations.tools':
+    handlers: ['default']
+    level: 'DEBUG'
+    propagate: false
+
+  'aprendizations.serve':
+    handlers: ['default']
+    level: 'DEBUG'
+    propagate: false
diff --git a/demo/astronomy/solar-system/questions.yaml b/demo/astronomy/solar-system/questions.yaml
index f77979e..c3fdbb7 100644
--- a/demo/astronomy/solar-system/questions.yaml
+++ b/demo/astronomy/solar-system/questions.yaml
@@ -28,6 +28,7 @@
     - Têm todos o mesmo tamanho
   # opcional
   correct: 2
+  # discount: true
   shuffle: false
   solution: |
     O maior planeta é Júpiter. Tem uma massa 1000 vezes inferior ao Sol, mas
diff --git a/demo/math/addition/questions.yaml b/demo/math/addition/questions.yaml
index 0ab7d95..6480bd4 100644
--- a/demo/math/addition/questions.yaml
+++ b/demo/math/addition/questions.yaml
@@ -5,6 +5,7 @@
   script: addition-two-digits.py
   args: [10, 20]
 
+# ---------------------------------------------------------------------------
 - type: checkbox
   ref: addition-properties
   title: Propriedades da adição
--
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