--- a/doc/book/en/development/datamodel/definition.rst	Tue Feb 02 21:14:01 2010 +0100
+++ b/doc/book/en/development/datamodel/definition.rst	Wed Feb 03 09:18:47 2010 +0100
@@ -397,6 +397,10 @@
 * it is possible to use the attribute `meta` to flag an entity type as a `meta`
   (e.g. used to describe/categorize other entities)
 
+*Note* : if you end up with an `if` in the definition of your entity, this probably
+means that you need two separate entities that implement the `ITree` interface and
+get the result from `.children()` which ever entity is concerned.
+
 Inheritance
 ```````````
 XXX feed me