Add R^d component affectation in affectation list

One can now write things like

``
R^3 x = (1,2,3);
R y = 0;

(x[0], x[1], x[2], y) = (y, x[0], x[1], x[2]);
``
After last instruction one will have `y==3` and `x==(0,1,2)`