What the heck does this backtrace mean?

[Stephen J. Turnbull]

Aidan Kehoe writes:
 > 
 >  Ar an cúigiú lá is fiche de mí Feabhra, scríobh David Kastrup: 
 > 
 >  > I'd prefer
 >  > M-x check-parens RET
 >  > 
 >  > It is less complicated and does not pollute the mark stack.
 > 
 > Something we should merge from GNU, then. 

Note that "polluting" the mark stack was deliberate; it is often the
case that a final paren is missing, but if not, C-x C-x will mark the
region where the error is most likely to be found.

If check-parens doesn't provide the same navigation functionality, we
should think about how to add it.  (Probably it's already much better,
but we should make sure of that.)