public static Behavior <TResult> Map <T, TResult>(this Behavior <T> b, Func <T, TResult> f) => b.MapImpl(f);
internal static Cell <T> SwitchCImpl <T, T2>(this Behavior <T2> bca) where T2 : Cell <T> => new Cell <T>(bca.MapImpl(c => c.BehaviorImpl).SwitchBImpl <T, Behavior <T> >());