Here's another version of the next-error patch.  What it does:

- move the next-error framework to simple.el, so it's always loaded.
  This was next-error plus all the related functions.

- make the next-error-function always local (but not buffer-local)

- hook compile.el, occur (in replace.el), and grep.el into the
  next-error framework; make it easy to hook more modes into the
  framework since it's guaranteed to be always loaded and using it is
  as simple as setting next-error-function to something meaningful to
  the mode

- preserve the compile.el behavior of:

a) if called in a usable buffer, use it for next-error
b) else, if given a usable buffer, use it
c) else, look through all the buffers and find a suitable one

- fix up some documentation

I hope it's nearly done :)  It seems to be useful in its current state.

Ted