public static Seq <A> Cons <A>(this A head) => SeqCons <A> .New(head, SeqEmpty <A> .Default);
public static Seq <A> Cons <A>(this A head, Seq <A> tail) => SeqCons <A> .New(head, tail);
public static Seq <A> Cons <A>(this A head, Arr <A> tail) => SeqCons <A> .New(head, SeqArr <A> .New(tail));