public static Seq <A> create <A>(params A[] items) => SeqArray <A> .New(items);
public static Seq <A> Cons <A>(this A head, A[] tail) => SeqArray <A> .New(head, tail);
public static Seq <A> Cons <A>(this A head, Arr <A> tail) => SeqCons <A> .New(head, SeqArray <A> .New(tail.Value));