public override Bookmark <ValueTuple <K, V> >?Next() // ok
{
var b = _bmk.Next();
return((b == null)? null:(Bookmark <ValueTuple <K, V> >) new SDictBookmark <K, V>(b));
}
public override Bookmark <ValueTuple <K, V> >?First()
{
return((SBookmark <K, V> .Next(null, this) is SBookmark <K, V> b)?
new SDictBookmark <K, V>(b):null);
}