Index: openacs-4/packages/categories/tcl/widget-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/categories/tcl/widget-procs.tcl,v
diff -u -N -r1.25 -r1.26
--- openacs-4/packages/categories/tcl/widget-procs.tcl	25 Apr 2018 19:47:46 -0000	1.25
+++ openacs-4/packages/categories/tcl/widget-procs.tcl	26 Apr 2018 08:56:37 -0000	1.26
@@ -127,7 +127,7 @@
     foreach mapped_tree $mapped_trees {
 	lassign $mapped_tree tree_id tree_name subtree_id assign_single_p require_category_p widget
 	set tree_name [ns_quotehtml [lang::util::localize $tree_name]]
-	set one_tree [list]
+	set one_tree {}
 
         if { $require_category_p == "t" } { 
             set required_mark "<span class=\"form-required-mark\">*</span>"
@@ -213,7 +213,7 @@
 
     # QUIRK: ns_querygetall returns a single-element list {{}} for no values
     if { [string equal $values {{}}] } {
-	set values [list]
+	set values {}
     }
 
     # to mark submission of form for rendering element in case of invalid data
@@ -250,7 +250,7 @@
     }
 
     if { $tree_id eq "" } {
-	set trees [list]
+	set trees {}
         foreach tree [category_tree::get_mapped_trees $package_id] {
 	    lassign $tree tree_id tree_name subtree_id assign_single_p require_category_p
 	    if {$require_category_p == "t" || ![info exists element(optional)]} {