/// <summary>
        /// Gets the level that will be associated to the SRS entry matching the
        /// given group in the current state.
        /// </summary>
        /// <returns>New level for the entity.</returns>
        private SrsLevel GetUpdatedLevel(SrsQuestionGroup group)
        {
            // Take the upper/lower level depending on the success state.
            int modifier = group.IsWrong ? -1 : 1;
            SrsLevel newLevel = newLevel = SrsLevelStore.Instance.GetLevelByValue(
                    group.Reference.CurrentGrade + modifier);

            // If the upper/lower level could not be obtained, (as this may
            // be the case when at max/min level), use the current one.
            // Note that this still may return a null value if the levels
            // were not loaded correctly or if the configuration has changed.
            return newLevel ?? SrsLevelStore.Instance.GetLevelByValue(
                    group.Reference.CurrentGrade);
        }
        /// <summary>
        /// Called to update the SRS entry referred by a question group that was
        /// answered.
        /// </summary>
        /// <param name="group">Question group answered.</param>
        private void UpdateAnsweredGroup(SrsQuestionGroup group)
        {
            // Get the new level of the item.
            SrsLevel newLevel = GetUpdatedLevel(group);

            // Update its grade and next answer date.
            if (newLevel != null)
            {
                // Set the next answer date according to the delay of the new level.
                if (newLevel.Delay.HasValue)
                {
                    group.Reference.NextAnswerDate =
                        DateTime.UtcNow
                        + newLevel.Delay.Value;
                }
                else
                {
                    group.Reference.NextAnswerDate = null;
                }

                group.Reference.CurrentGrade = newLevel.Value;
            }
            else
            {
                // If there is no new level and no current level, the delay is null.
                group.Reference.NextAnswerDate = null;
            }

            // Update the success/failure count.
            if (group.IsWrong)
            {
                group.Reference.FailureCount++;
            }
            else
            {
                group.Reference.SuccessCount++;
            }

            // Update the entry in the DB.
            UpdateEntry(group.Reference);
        }