This is interesting stuff.

I'm trying to figure out how to refer to flatMap directly from the abstract function, e.g.

    flatMapTwice(x, fun) {
        return x.flatMap(fun).flatMap(fun);
    }

or

    chain(x, fun1, fun2) {
        return x.flatMap(fun1).flatMap(fun2);
    }

It should also preserve the type information (just like you did above). I don't want the caller to need to downcast from FlatMappable.

It doesn't need to be x.flatMap(f) - I'd be interested seeing a flatMap(x, f) version.