Passing a reference to a returned reference

[Michael]

On Friday, 6 July 2018 at 15:14:01 UTC, Michael wrote:
> On Friday, 6 July 2018 at 14:50:39 UTC, Ali Çehreli wrote:
>> [...]
>
> I'm just trying to do that now.
>
> Here is what I have in terms of code:
>
> [...]

This is definitely to do with my use of the setter syntax, which 
maybe I am misunderstanding? Because if I change it to a normal 
function call like so:

a.beliefs(Operator.create());

then it complains if I use ref, and doesn't complain if I don't.