LaxUndefined

In many situations, it is preferable to hobble along (or even make only passing notice) when errors occur rather than blowing up the rendering completely.

My particular case is templates which are created by non-technical users in inkscape.  I have found over the last year or so that clear but self-contained effects are desirable in this role.

I do have a continuing issue though, where iteration occasionally trips over the concat implementation in utils;  runtime contains a suitable replacement (unicode_join becomes the initial _concat implementation instead of u"".join), but an alternative would be for LaxUndefined to inherit from unicode instead of Undefined, and registering itself on Undefined's new metaclass.  I'm not sure which you would find more palatable.

Cheers
--cwillu