public TimePeriodList SelectTimePeriodsRecursive(DateTime since, DateTime till)
{
var nodes = Childs.SelectRecursiveList(n => n.Childs);
nodes.Insert(0, this);
TimePeriodList periods = new TimePeriodList();
foreach (var node in nodes)
{
periods.AddRange(node.TimePeriods.Where(p => p.Since > since && p.Till < till));
}
periods.Sort(delegate(TimePeriod a, TimePeriod b) { return(a.Since < b.Since ? -1 : 1); });
return(periods);
}