diff --git a/BUGS.md b/BUGS.md
index 3da7246..81b6095 100644
--- a/BUGS.md
+++ b/BUGS.md
@@ -14,7 +14,6 @@
 
 # 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.
 - ao fim de 3 tentativas com password errada, envia email com nova password.
@@ -35,6 +34,7 @@
 
 # FIXED
 
+- 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.
 - 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.
diff --git a/aprendizations/questions.py b/aprendizations/questions.py
index 689f913..474860a 100644
--- a/aprendizations/questions.py
+++ b/aprendizations/questions.py
@@ -113,8 +113,8 @@ class QuestionRadio(Question):
             # 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"]}"')
+                msg = (f'Correct values must be in the interval [0.0, 1.0] in '
+                       f'"{self["ref"]}"')
                 raise QuestionException(msg)
 
             # at least one correct option
@@ -217,12 +217,18 @@ class QuestionCheckbox(Question):
                    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)
+        # check grade boundaries
+        if self['discount'] and not all(0.0 <= x <= 1.0
+                                        for x in self['correct']):
+            logger.warning('          * * *  BEHAVIOR CHANGE NOTICE  * * *')
+            msg = (f'Correct values must be in the interval [0.0, 1.0] in '
+                   f'"{self["ref"]}"')
+            logger.warning(msg)
+            logger.warning('I will convert "correct" to the new behavior, but '
+                           'you should change it in the questions')
+            logger.warning('          * * *  END OF NOTICE           * * *')
+            # normalize to [0,1]
+            self['correct'] = [(x+1)/2 for x in self['correct']]
 
         # if an option is a list of (right, wrong), pick one
         options = []
@@ -232,9 +238,10 @@ class QuestionCheckbox(Question):
                 r = random.randint(0, 1)
                 o = o[r]
                 if r == 1:
-                    c = -c
+                    # c = -c
+                    c = 1.0 - c
             options.append(str(o))
-            correct.append(float(c))
+            correct.append(c)
 
         # generate random permutation, e.g. [2,1,4,0,3]
         # and apply to `options` and `correct`
@@ -252,21 +259,20 @@ class QuestionCheckbox(Question):
         super().correct()
 
         if self['answer'] is not None:
-            sum_abs = sum(abs(p) for p in self['correct'])
-            if sum_abs < 1e-6:  # case correct [0,...,0] avoid div-by-zero
-                self['grade'] = 1.0
-
+            x = 0.0
+            if self['discount']:
+                sum_abs = sum(abs(2*p-1) for p in self['correct'])
+                for i, p in enumerate(self['correct']):
+                    x += 2*p-1 if str(i) in self['answer'] else 1-2*p
             else:
-                x = 0.0
-
-                if self['discount']:
-                    for i, p in enumerate(self['correct']):
-                        x += p if str(i) in self['answer'] else -p
-                else:
-                    for i, p in enumerate(self['correct']):
-                        x += p if str(i) in self['answer'] else 0.0
+                sum_abs = sum(abs(p) for p in self['correct'])
+                for i, p in enumerate(self['correct']):
+                    x += p if str(i) in self['answer'] else 0.0
 
+            try:
                 self['grade'] = x / sum_abs
+            except ZeroDivisionError:
+                self['grade'] = 1.0   # limit p->0
 
 
 # ============================================================================
diff --git a/demo/math/addition/questions.yaml b/demo/math/addition/questions.yaml
index 6480bd4..903dace 100644
--- a/demo/math/addition/questions.yaml
+++ b/demo/math/addition/questions.yaml
@@ -18,6 +18,6 @@
     - Propriedade comutativa, $x+y=y+x$.
     # wrong
     - Existência de elemento absorvente, $x+1=1$.
-  correct: [1, 1, 1, 1, -1]
+  correct: [1, 1, 1, 1, 0]
   solution: |
     A adição não tem elemento absorvente.
--
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