diff --git a/questions.py b/questions.py
index 6a65d0a..bd29b11 100644
--- a/questions.py
+++ b/questions.py
@@ -1,22 +1,25 @@
 
 # Example:
 #
+#   # read everything from question files
 #   pool = QuestionPool()
 #   pool.add_from_files(['file1.yaml', 'file1.yaml'])
 #
+#   # generate a new test, creating instances for all questions
 #   test = []
 #   for q in pool.values():
 #       test.append(create_question(q))
 #
+#   # experiment answering one question and correct it
 #   test[0]['answer'] = 42           # insert answer
 #   grade = test[0].correct()        # correct answer
 
 
-# Functions:
-#    create_question(q)
-#           q       - dictionary with question in yaml format
-#           returns - question instance with the correct class
 
+#    QuestionsPool - dictionary of questions not yet instantiated
+#
+#    question_generator - runs external script to get a question dictionary
+#    create_question - returns question instance with the correct class
 
 # An instance of an actual question is a Question object:
 #
@@ -28,12 +31,18 @@
 # QuestionTextArea  - corrected by an external program
 # QuestionInformation - not a question, just a box with content
 
-import yaml
 import random
 import re
 import subprocess
 import os.path
 import logging
+import sys
+
+try:
+    import yaml
+except ImportError:
+    print('The package "yaml" is missing. See README.md for instructions.')
+    sys.exit(1)
 
 
 qlogger = logging.getLogger('Questions')
diff --git a/serve.py b/serve.py
index be598c4..cbad33d 100755
--- a/serve.py
+++ b/serve.py
@@ -4,10 +4,23 @@
 # The program runs a web server where students login to answer a sequence of questions.
 # Their grades are automatically calculated and stored in a sqlite3 database.
 
-import cherrypy
-from mako.lookup import TemplateLookup
-import argparse
 from os import path
+import sys
+import argparse
+# from threading import Lock
+
+try:
+    import cherrypy
+except ImportError:
+    print('The package "cherrypy" is missing. See README.md for instructions.')
+    sys.exit(1)
+
+try:
+    from mako.lookup import TemplateLookup
+    import djks
+except ImportError:
+    print('The package "mako" is missing. See README.md for instructions.')
+    sys.exit(1)
 
 # path where this file is located
 SERVER_PATH = path.dirname(path.realpath(__file__))
@@ -30,7 +43,9 @@ class Root(object):
         self.database = database.Database(testconf['database'])
         self.auth = AuthController(database=testconf['database'])
         self.templates = TemplateLookup(directories=[TEMPLATES_DIR], input_encoding='utf-8')
-        self.tags = {'online': set(), 'finished': set()}  # FIXME should be in application, not server
+        # FIXME should be in application, not server
+        # FIXME lock and threading
+        self.tags = {'allowed': set(['0']), 'online': set(), 'finished': set()}
 
     # --- DEFAULT ------------------------------------------------------------
     # any path, e.g. /xpto/aargh is redirected to the test
@@ -89,6 +104,12 @@ class Root(object):
         # If it's the first time, create instance of the test and register the
         # time.
         uid = cherrypy.session.get('userid')
+        # if uid not in self.tags['allowed']:
+        #     print('not allowed')
+        #     raise cherrypy.HTTPRedirect('/logout')
+        #     return
+        # print(self.tags)
+
         name = cherrypy.session.get('name')
         t = cherrypy.session.get('test', None)
         if t is None:
diff --git a/test.py b/test.py
index e067838..fd2eed5 100644
--- a/test.py
+++ b/test.py
@@ -1,10 +1,22 @@
 
 import os, sys, fnmatch
 import random
-import yaml, json
 import sqlite3
 from datetime import datetime
 
+try:
+    import yaml
+except ImportError:
+    print('The package "yaml" is missing. See README.md for instructions.')
+    sys.exit(1)
+
+try:
+    import json
+except ImportError:
+    print('The package "json" is missing. See README.md for instructions.')
+    sys.exit(1)
+
+
 # my code
 import questions
 import database
--
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