Please, put this example on the wiki (together with some sample images
that can be created, with or without some cheating).

The community would be greatful for that.

> Hans, Taco, et al,
>
> This is getting pretty close to the limit of my expertise. The good
> stuff of it is stolen from Metafun and mp-func.mp.
>
> Because of the way I handle clipping the path,
>
>    p:=p cutbefore cpath;
>    p:=reverse p;
>    p:=p cutbefore cpath;
>
> this will only work on functions (like lines, quadratics, and
> polynomials, maybe sinusoids) that both enter and leave the region
> inside the clipping path.

If I understand it properly (that you only want to draw the content
inside the square, while the function may be a circle or something
lying completely outside the region), you could also use something
like
    clip currentpicture to cpath;
although you may have to change the code that draws double arrow then.

Mojca