Tree does not need to have a field inside.

Update form.rng with trang.
issue2535

trytond/ir/ui/form.rnc

@@ -20,7 +20,7 @@ attlist.form &= attribute on_write { text }?
 attlist.form &= [ a:defaultValue = "4" ] attribute col { text }?
 attlist.form &= attribute cursor { text }?
 
-tree = element tree { attlist.tree, field+ }
+tree = element tree { attlist.tree, field }
 attlist.tree &=
   [ a:defaultValue = "Unknown" ] attribute string { text }?
 attlist.tree &= attribute on_write { text }?

trytond/ir/ui/form.rng

@@ -42,9 +42,7 @@
   <define name="tree">
     <element name="tree">
       <ref name="attlist.tree"/>
-      <oneOrMore>
-        <ref name="field"/>
-      </oneOrMore>
+      <ref name="field"/>
     </element>
   </define>
   <define name="attlist.tree" combine="interleave">
@@ -500,11 +498,6 @@
       </attribute>
     </optional>
   </define>
-  <define name="attlist.image" combine="interleave">
-    <optional>
-      <attribute name="help"/>
-    </optional>
-  </define>
   <define name="attlist.image" combine="interleave">
     <optional>
       <attribute name="xexpand" a:defaultValue="0">
@@ -525,6 +518,11 @@
       </attribute>
     </optional>
   </define>
+  <define name="attlist.image" combine="interleave">
+    <optional>
+      <attribute name="help"/>
+    </optional>
+  </define>
   <define name="attlist.image" combine="interleave">
     <optional>
       <attribute name="states"/>