Generic 'append' returning <gtk-list-store> in 'editable-cells.scm'

The patch at the bottom fix a problem with the example
'editable-cells.scm', but I think that maybe there's a bigger problem
to solve.

Apparently the generic method 'append' is returning a <gtk-list-store>
and not a <gtk-tree-iter>, which would be expected.

Unfortunately I didn't find where the generic method is define.

As a side note, there's a unused column in the model.

diff --git a/examples/guile-gtk-demo/demos/editable-cells.scm b/examples/guile-gtk-demo/demos/editable-cells.scm
index 78dae68..bcee16a 100644
--- a/examples/guile-gtk-demo/demos/editable-cells.scm
+++ b/examples/guile-gtk-demo/demos/editable-cells.scm
@@ -17,7 +17,7 @@
      (6 "eggs" #t))))
 
 (define (add-item model item)
-  (let ((iter (append model)))
+  (let ((iter (gtk-list-store-append model)))
     (for-each
      (lambda (i x) (set-value model iter i x))
      '(0 1 2)

-- 
Diogo F. S. Ramos