How to do multiple key join in Cubert?

[Jiaqi Luo]

I know Using the JOIN or MESH-JOIN can do single key join. For example:

joined = JOIN relation1 by field1, relation2 by field1;

Now I want to do the multiple key join like

joined = JOIN relation1 by (field1, filed2), relation 2 by (filed1, field2);

But this syntax is not correct~~

Anyone has any idea about it?

Thanks so much in advance.