doc: Add a REPL example.

* doc/contributing.texi (Running Guix Before It Is Installed): Add REPL
  example.
* doc/emacs.texi (Emacs General info): Add @cindex.

doc/contributing.texi

@@ -86,6 +86,30 @@ Similarly, for a Guile session using the Guix modules:
 
 @example
 $ ./pre-inst-env guile -c '(use-modules (guix utils)) (pk (%current-system))'
+
+;;; ("x86_64-linux")
+@end example
+
+@noindent
+@cindex REPL
+@cindex read-eval-print loop
+@dots{} and for a REPL (@pxref{Using Guile Interactively,,, guile, Guile
+Reference Manual}):
+
+@example
+$ ./pre-inst-env guile
+scheme@@(guile-user)> ,use(guix)
+scheme@@(guile-user)> ,use(gnu)
+scheme@@(guile-user)> (define snakes
+                       (fold-packages
+                         (lambda (package lst)
+                           (if (string-prefix? "python"
+                                               (package-name package))
+                               (cons package lst)
+                               lst))
+                         '()))
+scheme@@(guile-user)> (length snakes)
+$1 = 361
 @end example
 
 The @command{pre-inst-env} script sets up all the environment variables

doc/emacs.texi

@@ -227,6 +227,8 @@ prefix argument is used.  This has the same meaning as @code{--manifest}
 option (@pxref{Invoking guix package}).
 
 @item C-c C-z
+@cindex REPL
+@cindex read-eval-print loop
 Go to the Guix REPL (@pxref{The REPL,,, geiser, Geiser User Manual}).
 
 @item h