diff --git a/ostap/parallel/task.py b/ostap/parallel/task.py
index adfef39c..a213a435 100644
--- a/ostap/parallel/task.py
+++ b/ostap/parallel/task.py
@@ -36,8 +36,8 @@
 __all__     = (
     'Task'          , ## the base class for task
     'TaskManager'   , ## the base class for task-manager 
-    'GenericTask'   , ## the generic ``templated'' task
-    'FuncTask'      , ## the simple ``function'' task
+    'GenericTask'   , ## the generic `templated' task
+    'FuncTask'      , ## the simple `function' task
     'Statistics'    , ## helper class to collect statistics 
     'StatMerger'    , ## helper class to merge   statistics
     'TaskMerger'    , ## simple merger for task results
@@ -64,8 +64,8 @@
 #  One can specify following attributes:
 #  - <code>directory</code>: the working directory for the job
 #  - <code>environment</code>: additional environmental variables 
-#  - <code>append_to</code>: append some path-like enviroment varibales 
-#  - <code>prepend_to</code>: prepend some path-like enviroment varibales 
+#  - <code>append_to</code>: append some path-like environment varibales 
+#  - <code>prepend_to</code>: prepend some path-like environment varibales 
 #  - <code>dot_in_path</code>: shoud the '.' be added to sys.path?
 #  @author Pere MATO Pere.Meto@cern.ch
 class Task(object) :
@@ -77,10 +77,10 @@ class Task(object) :
     - process
     - finalize.
     One can specify following attributes:
-    - <directory : the working directory for the job
+    - directory : the working directory for the job
     - environment : additional environmental variables 
-    - append_to : append some path-like enviroment varibales 
-    - prepend_to : prepend some path-like enviroment varibales 
+    - append_to : append some path-like environment varibales 
+    - prepend_to : prepend some path-like environment varibales 
     - dot_in_path : shoud the '.' be added to sys.path?
     """
     __metaclass__ = abc.ABCMeta
@@ -155,12 +155,12 @@ def __call__          ( self , jobid , *params ) :
 
     @property
     def output  ( self ) :
-        """`output' : get a task output (it invokes the method `results` """
+        """`output' : get a task output (internally it invokes the method `results` """
         return self.results()
 
     @property
     def directory ( self ) :
-        """``Directory'' : directory where job starts"""
+        """`Directory' : directory where job starts"""
         return self.__directory
     
     @directory.setter 
@@ -169,12 +169,12 @@ def directory ( self , value ) :
 
     @property
     def environment ( self ) :
-        """``environment'' : additional environment for the job"""
+        """`environment' : additional environment for the job"""
         return self.__environment
     
     @environment.setter
     def environment ( self , value ) :
-        self.__environment.update (value ) 
+        self.__environment.update ( value ) 
     
     @property
     def append_to ( self ) :
@@ -234,7 +234,7 @@ def cleanup ( self , value ) :
         
 # =============================================================================
 ## @class GenericTask
-#  Generic ``templated'' task for Parallel processing  
+#  Generic `templated' task for Parallel processing  
 #  One needs to  define three functions/functors:
 #    - processor   :<code>        output = processor ( jobid , item )         </code>
 #    - merger      :<code>updated_output = merger ( old_output , new_output ) </code>
@@ -243,7 +243,7 @@ def cleanup ( self , value ) :
 #    - append_to   : additional variables to be ''appended''
 #    - prepend_to  : additional variables to be ''prepended''
 class GenericTask(Task) :
-    """Generic ``templated'' task for parallel processing.
+    """Generic `templated' task for parallel processing.
     One needs to  define three functions/functors:
     - processor   :         output = processor   ( jobid , item ) 
     - merger      : updated_output = merger    ( old_output , new_output )
@@ -280,6 +280,12 @@ def __init__ ( self                ,
             import operator
             merger = operator.add
 
+        assert processor , "GenericTask: `processor' it not defiend!"
+
+        assert merger or collector , \
+            "GenericTask: neither `merger' nor `collector' are defined!"
+        
+        
         self.__processor   = processor
         self.__merger      = merger
         self.__collector   = collector
@@ -292,7 +298,7 @@ def __init__ ( self                ,
         self.prepend_to   . update ( prepend_to  )
         
         self.cleanup       = cleanup
-
+        
         
     # =========================================================================
     ## local initialization (executed once in parent process)
@@ -324,25 +330,27 @@ def results ( self ) :
     # =========================================================================
     @property
     def processor  ( self ) :
-        """``processor'' : the actual function for each subprocess
+        """`processor' : the actual function for each subprocess
         - Signature: output = processor ( item ) 
         """
         return self.__processor
+    
     @property
     def merger     ( self ) :
-        """``merger'' : the actual fuction to merge results
+        """`merger' : the actual function to merge results
         - Signature: updated_output = merger ( old_output , new_output )         
         """
         return self.__merger
+    
     @property
     def collector  ( self ) :
-        """``collector'' : the actual fuction to merge/collect results
+        """`collector' : the actual function to merge/collect results
         - Signature: updated_output = collector ( old_output , new_output , jobid )         
         """
         return self.__collector
     @property
     def initializer ( self ) :
-        """``initializer'' : the actual fuction to initialize local output  
+        """`initializer' : the actual function to initialize local output  
         - Signature: output = initializer() 
         """
         return self.__initializer
@@ -411,13 +419,14 @@ def results ( self ) :
 
     @property
     def merger     ( self ) :
-        """``merger'' : the actual fuction to merge results
+        """`merger' : the actual fuction to merge results
         - Signature: updated_output = merger ( old_output , new_output )         
         """
         return self.__merger
+    
     @property
     def initializer ( self ) :
-        """``initializer'' : the actual fuction to initialize local output  
+        """`initializer' : the actual fuction to initialize local output  
         - Signature: output = initializer() 
         """
         return self.__initializer
@@ -450,7 +459,7 @@ def __exit__  ( self , *_ ) : self.stop()
     
     @property 
     def host ( self  ) :
-        """``host'' :  the host where the job executed"""
+        """`host' :  the host where the job executed"""
         return self.__host
 
     def __repr__  ( self ) :
@@ -503,7 +512,7 @@ def __iter__  ( self ) :
             
     @property
     def merged ( self ) :
-        """``merged'' : get the full merged statistic"""
+        """`merged' : get the full merged statistic"""
         return self.__merged 
 
     # =========================================================================
@@ -593,7 +602,7 @@ def __str__  ( self ) :
 
     @property 
     def njobs ( self ) :
-        """``njobs'' : total number of jobs"""
+        """`njobs' : total number of jobs"""
         return sum ( s.njobs for s in self.__merged.values() ) 
     
     __repr__ = __str__
@@ -679,12 +688,12 @@ def merge ( self , result ) :
         
     @property
     def result  ( self ) :
-        """``result'' : the merged results"""
+        """`result' : the merged results"""
         return self.__result
     
     @property
     def nmerged  ( self ) :
-        """``nmerged'' : number of merged results"""
+        """`nmerged' : number of merged results"""
         return self.__nmerged
 
     def __nonzero__ ( self ) : return 0 < self.__nmerged
@@ -698,7 +707,7 @@ def __len__     ( self ) : return     self.__nmerged
 #  @see Task 
 def task_executor ( item ) :
     """Helper function to execute the task and collect job execution statistic
-    - unfortunately due to limitation of ``parallel python'' one cannot
+    - unfortunately due to limitation of `parallel python' one cannot
     use python decorators here :-(
     - see Task 
     """
@@ -788,7 +797,7 @@ def task_executor ( item ) :
 #  use decorators here :-(
 def func_executor ( item ) :
     """Helper function to execute the task and collect job execution statistic
-    - unfornately due to limitation of ``parallel python'' one cannot
+    - unfornately due to limitation of `parallel python' one cannot
     use python decorators here :-(
     """
     ## unpack
@@ -893,9 +902,9 @@ def __process_func ( self , task , chunks  , **kwargs ) :
         
         ## mergers for statistics & results
         if   not merger and not collector :
-            logger.warning ( "Neither ``merger'' nor ``collector'' are specified for merging!")
+            logger.warning ( "Neither `merger' nor `collector' are specified for merging!")
         elif     merger and     collector :
-            logger.warning ( "Both    ``merger'' and ``collector'' are specified for merging!")
+            logger.warning ( "Both    `merger' and `collector' are specified for merging!")
             
         ## mergers for statistics 
         merged_stat    = StatMerger ()
@@ -996,17 +1005,17 @@ def __process_task  ( self , task , chunks , **kwargs ) :
     
     @property
     def silent ( self ) :
-        """``silent'' : silent processing?"""
+        """`silent' : silent processing?"""
         return self.__silent
 
     @property
     def progress ( self ) :
-        """``progress'' : show progress bar?"""
+        """`progress' : show progress bar?"""
         return self.__progress
 
     @property
     def ncpus ( self ) :
-        """``ncpus'' : number of CPUs"""
+        """`ncpus' : number of CPUs"""
         return self.__ncpus
 
     # ===========================================================================
@@ -1043,9 +1052,9 @@ def iexecute ( self , job , jobs_args , progress = False ) :
         >>> for result in mgr.iexecute ( job , args ) :
         ...
         ...
-        It is a ``minimal'' interface
+        It is a `bare minimal' interface
         - no statistics
-        - no summary prin
+        - no summary printout
         - no merging of results  
         """
         return None