[Hat] Hat bug report: hat-stack fails on simple example

[Malcolm Wallace]

I wrote:

> Perhaps one day we will rewrite hat-stack in Haskell as a cut-down
> version of hat-trail.

And realised that today is the day...  Well, all the library code for
traversing the trace, and for printing trace expressions is already
there, so it is just a matter of running a little loop really.  So,
a new implementation of hat-stack has just been committed to CVS.

Regards,
    Malcolm