issue with fi-stella.el with proposed patch

I was just working on a CL program that would interface with
PowerLoom, and encountered some problems with my development
environment.  When I try to find definitions for a CL function, the
Allegro ELI environment tosses an error.  AFAICT, the problem seems to
be that no matter whether I'm trying to find a stella definition or
not, the fi::show-found-definition advice in fi-stella.el steps in and
turns a perfectly servicable filename returned by the Allegro lisp
environment into a nonsensical logical pathname.

The problem is that as far as I can tell, fi::show-found-definition
will invoke stella-file-name *whether or not you have stella loaded*,
once you load fi-stella.el.  I.e., if I load fi-stella.el in my .emacs
file, then go on to just work with CL code, then when I use meta-. it
will crash ELI because the "PL:" logical pathname host is undefined.

This means that the stella ELI development can't coexist with vanilla
CL environment.

I believe it would be more elegant if one had some way of stashing
away information about whether a given definition is Stella or
Lisp-native, but I don't know how to do that.  In the absence of this,
here's a proposed patch to fi-stella that makes it coexist better:

;;; Wrap these two forms to evaluate in IGNORE-ERRORS so that they
;;; just return NIL, instead of crashing the emacs interface:

(defvar fi-stella-lisp-directory-form
  ;; Form to evaluate to determine the directory where Lisp translations of
  ;; STELLA files are stored.  We can't evaluate this at initialization time,
  ;; since STELLA might not yet be up and running.
  ;; Not sure how to do this right, since the Lisp translations could really
  ;; go anywhere dependent on the pathname definitions for a particular system.
  ;; Maybe these should be lists, or should we store the STELLA source file in
  ;;    the Lisp translation instead?
  '(fi:eval-in-lisp
    "(cl:ignore-errors (CL:namestring (CL:translate-logical-pathname \"PL:native;lisp;\")))"))

(defvar fi-stella-sources-directory-form
  ;; Form to evaluate to determine the directory where STELLA sources
  ;; are stored.
  '(fi:eval-in-lisp
    "(cl:ignore-errors (CL:namestring (CL:translate-logical-pathname \"PL:sources;\")))"))

;;; Modify STELLA-FILE-NAME to give up if it can't find the
;;; stella-lisp-directory or stella-sources-directory.  See the catch
;;; and throw forms...
(defun stella-file-name (lisp-file-name)
  ;; Return the Stella source file of `lisp-file-name' if it exists.
  (catch 'no-logical-pathname
    (let* ((lisp-name-end
	    (and (stringp lisp-file-name)
		 (or (string-match "\\.lisp$" lisp-file-name)
		     (string-match "\\.slisp$" lisp-file-name)
		     (string-match "\\.vslisp$" lisp-file-name))))
	   (lisp-file-directory
	    (file-name-directory lisp-file-name))
	   (basename-sans-extension
	    (file-name-sans-extension (file-name-nondirectory lisp-file-name)))
	   (lisp-directory (or (eval fi-stella-lisp-directory-form)
			       (throw 'no-logical-pathname nil)))
	   (sources-directory (or (eval fi-stella-sources-directory-form)
				  (throw 'no-logical-pathname nil)))
	   (stella-file-name nil)
	   ;; Directories to try to find the STELLA source:
	   (directories
	    (list lisp-file-directory
		  (concat sources-directory
			  (substring lisp-file-directory
				     (length lisp-directory))))))
      (cond
	(lisp-name-end
	 (while directories
	   (setq stella-file-name
		 (concat (car directories) basename-sans-extension ".ste"))
	   (if (file-exists-p stella-file-name)
	       (setq directories nil)
	       (setq stella-file-name nil)
	       (setq directories (cdr directories))))
	 stella-file-name)))))