diff --git a/models/dimers random walk.xml b/models/dimers random walk.xml
index c5696fd..c6a063d 100644
--- a/models/dimers random walk.xml	
+++ b/models/dimers random walk.xml	
@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="UTF-8"?>
+<?xml version="1.0" encoding="UTF-8"?>
 <gem-graph-model version="0.2">
     <!-- Model identity informations -->
     <identity>
@@ -24,8 +24,15 @@
         </simulation>
 
         <space-param>
-            <dimension>1</dimension>
-            <size x="29" y="0" z="0"/>
+            <dimension>
+    <!-- loop_on_zero="true"  anything moved before zero reenters at the end of space-->
+    <!-- loop_on_zero="false" anything moved before zero is lost-->
+    <!-- loop_on_max="true"   anything moved beyond max reenters at the zero of space-->
+    <!-- loop_on_max="false"  anything moved beyond max is lost-->
+                <size x="29" loop_on_zero="true" loop_on_max="true"/>
+                <size y="0" loop_on_zero="true" loop_on_max="true"/>
+                <size z="0" loop_on_zero="true" loop_on_max="true"/>
+            </dimension>
             <!-- Site_multiplicity = number of  sites in a space unit. -->
             <!-- Each site points towards a neighbouring space unit. -->
             <!-- Several arrows can be stacked in the same site. -->