public List <SnailNumber> NonRegularNumbersList()
{
if (IsRegular)
{
return(new List <SnailNumber>());
}
if (Item1.IsRegular || Item2.IsRegular)
{
return(new List <SnailNumber> {
this
}.Concat(Item1.NonRegularNumbersList()).Concat(Item2.NonRegularNumbersList()).ToList());
}
return(Item1.NonRegularNumbersList().Concat(Item2.NonRegularNumbersList()).ToList());
}