code: def setOffset(self, value):
          
          if value >= len(self.data.getvalue()):
              raise excep.WrongOffsetValueException("Wrong offset value. Must be less than %d" % len(self.data))
          self.data.seek(value)
docs: Sets the offset of the L{WriteData} stream object in wich the data is written.
          
          @type value: int
          @param value: Integer value that represent the offset we want to start writing in the L{WriteData} stream.
              
          @raise WrongOffsetValueException: The value is beyond the total length of the data.