/// <summary>
        /// Returns or forces the next choice to schedule.
        /// </summary>
        /// <param name="next">Next</param>
        /// <param name="choices">Choices</param>
        /// <param name="current">Curent</param>
        /// <returns>Boolean</returns>
        private bool GetNextHelper(ref ISchedulable next, List <ISchedulable> choices, ISchedulable current)
        {
            ++ScheduledSteps;
            switch (Reduction)
            {
            case ReductionStrategy.ForceSchedule:
            {
                var partialOrderChoices =
                    choices
                    .Where(choice => choice.IsEnabled && IsPartialOrderOperation(choice.NextOperationType))
                    .ToList();

                // If we are being forced:
                if (next != null)
                {
                    if (!partialOrderChoices.Contains(next))
                    {
                        // Tell child strategy that we were forced (to do a particular send).
                        ChildStrategy.ForceNext(next, choices, current);
                        return(true);
                    }

                    // We would have forced this choice anyway so don't tell ChildStrategy.
                    return(true);
                }

                // Not being forced:
                if (partialOrderChoices.Count > 0)
                {
                    // Force this non-send but don't tell ChildStrategy.
                    next = partialOrderChoices[0];
                    return(true);
                }

                // Normal schedule:
                return(ChildStrategy.GetNext(out next, choices, current));
            }

            case ReductionStrategy.OmitSchedulingPoints:
            {
                // Otherwise, don't schedule before non-Send.

                bool continueWithCurrent =
                    current.IsEnabled &&
                    IsPartialOrderOperation(current.NextOperationType);

                // We are being forced:
                if (next != null)
                {
                    // ...to do something different than we would have:
                    if (continueWithCurrent && current != next)
                    {
                        // ...so tell child.
                        ChildStrategy.ForceNext(next, choices, current);
                        return(true);
                    }
                    // Otherwise, don't tell child.
                    return(true);
                }

                // Not being forced:

                if (continueWithCurrent)
                {
                    next = current;
                    return(true);
                }

                // Normal schedule:
                return(ChildStrategy.GetNext(out next, choices, current));
            }

            case ReductionStrategy.None:
            {
                // Normal schedule:
                if (next != null)
                {
                    ChildStrategy.ForceNext(next, choices, current);
                    return(true);
                }
                return(ChildStrategy.GetNext(out next, choices, current));
            }

            default:
                throw new ArgumentOutOfRangeException();
            }
        }