public bool Contains(v value) => WBT <v> .Lookup(WBT <v> .MkComp(_ord, value), _tree).IsSome();
public Opt <v> Lookup(k key) => WBT <Pair <k, v> > .Lookup(WBT <k> .MkComp(_ord, key).CoMap <Pair <k, v>, k, Ord>(Dtors.Left), _tree).Map(kv => kv.Right());
