Why are opCall's not implicitely assignable?

[Karen Lanrap]

Derek Parnell wrote:

>   with car.roof
>   {
>      .topcoat = COLOR.RED;
>      .undercoat = COLOR.WHITE;
>   }
> 
> is nice too.

Until someone says: topcoat is _not_ a color. It is concrete material 
that surely must have a property called color, but it is not a color 
itself: this code is totally unreadable for me.

(By the way: forgetting to delete a point can have desastrous results 
in D.)