public static async Task <Lst <B> > Traverse <A, B>(this Lst <Task <A> > ma, Func <A, B> f)
{
var rb = await Task.WhenAll(ma.Map(async a => f(await a)));
return(new Lst <B>(rb));
}
public static HashSet <Lst <B> > Traverse <A, B>(this Lst <HashSet <A> > ma, Func <A, B> f) =>
toHashSet(CollT.AllCombinationsOf(ma.Map(xs => xs.ToList()).ToSystemArray(), f)
.Map(toList));
public static Arr <Lst <B> > Sequence <A, B>(this Lst <A> ta, Func <A, Arr <B> > f) => ta.Map(f).Sequence();