We can't set, e.g. `__radd__ = NotImplemented` as it's not a callable.
NotImplemented is what is returned from a rich comparison method
when self doesn't know how to compare with the other object.
_LazyList object may also occur on the right hand side of a `+`
operator, when the left operand is a list. Also in this case we
want to first unpack the _LazyList and return a new list containing
elements from both.