public static Seq <T> append <T>(Seq <T> lhs, Seq <T> rhs) => lhs.Concat(rhs);
public SeqConcat <A> AddSeqRange(Seq <ISeqInternal <A> > ma) => new SeqConcat <A>(ms.Concat(ma));
public static Seq <T> append <T>(Seq <T> lhs, Seq <T> rhs) => lhs.IsEmpty ? rhs : lhs.Tail.IsEmpty ? lhs.Head.Cons(rhs) : Seq(lhs.Concat(rhs));