internal override IEnumerable <String> ExtractNames()
{
if (First != null)
{
foreach (var n in First.ExtractNames())
{
yield return(n);
}
}
if (Second != null)
{
foreach (var n in Second.ExtractNames())
{
yield return(n);
}
}
if (Last != null)
{
foreach (var n in Last.ExtractNames())
{
yield return(n);
}
}
}