static Seq <B> BindLazy <B>(Seq <A> ma, Func <A, Seq <B> > f) => Seq(EnumerableOptimal.BindFast(ma, a => f(a).AsEnumerable()));
static IEnumerable <B> BindLazy <B>(IEnumerable <A> ma, Func <A, IEnumerable <B> > f) => EnumerableOptimal.BindFast(ma, f);