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+
+Class `TreeMixIn`
+-----------------
+
+This class provides a tree interface. This mixin has to be inherited 
+explicitly and configured using the tree_attribute, parent_target and 
+children_target class attribute to benefit from this default implementation.
+
+This class provides the following methods:
+
+  * `different_type_children(entities=True)`, returns children entities
+    of different type as this entity. According to the `entities` parameter, 
+    returns entity objects (if entity=True) or the equivalent result set.
+
+  * `same_type_children(entities=True)`, returns children entities of 
+    the same type as this entity. According to the `entities` parameter, 
+    return entity objects (if entity=True) or the equivalent result set.
+  
+  * `iterchildren( _done=None)`, iters on the children of the entity.
+  
+  * `prefixiter( _done=None)`
+  
+  * `path()`, returns the list of eids from the root object to this object.
+  
+  * `iterparents()`, iters on the parents of the entity.
+  
+  * `notification_references(view)`, used to control References field 
+    of email send on notification for this entity. `view` is the notification view.
+    Should return a list of eids which can be used to generate message ids
+    of previously sent email.
+
+`TreeMixIn` implements also the ITree interface (``cubicweb.interfaces``):
+
+  * `parent()`, returns the parent entity if any, else None (e.g. if we are on the
+    root)
+
+  * `children(entities=True, sametype=False)`, returns children entities
+    according to the `entities` parameter, return entity objects or the
+    equivalent result set.
+
+  * `children_rql()`, returns the RQL query corresponding to the children
+    of the entity.
+
+  * `is_leaf()`, returns True if the entity does not have any children.
+
+  * `is_root()`, returns True if the entity does not have any parent.
+
+  * `root()`, returns the root object of the tree representation of
+    the entity and its related entities.
+
+Example of use
+``````````````
+
+Imagine you defined three types of entities in your schema, and they
+relates to each others as follows in ``schema.py``::
+
+  class Entity1(EntityType):
+      title = String()
+      is_related_to = SubjectRelation('Entity2', 'subject')
+
+  class Entity2(EntityType):
+      title = String()
+      belongs_to = SubjectRelation('Entity3', 'subject')
+
+  class Entity3(EntityType):
+      name = String()
+
+You would like to create a view that applies to both entity types
+`Entity1` and `Entity2` and which lists the entities they are related to.
+That means when you view `Entity1` you want to list all `Entity2`, and
+when you view `Entity2` you want to list all `Entity3`.
+
+In ``entities.py``::
+
+  class Entity1(TreeMixIn, AnyEntity):
+      id = 'Entity1'
+      __implements__ = AnyEntity.__implements__ + (ITree,)
+      __rtags__ = {('is_related_to', 'Entity2', 'object'): 'link'}
+      tree_attribute = 'is_related_to'
+
+      def children(self, entities=True):
+          return self.different_type_children(entities)
+
+  class Entity2(TreeMixIn, AnyEntity):
+      id = 'Entity2'
+      __implements__ = AnyEntity.__implements__ + (ITree,)
+      __rtags__ = {('belongs_to', 'Entity3', 'object'): 'link'}
+      tree_attribute = 'belongs_to'
+
+      def children(self, entities=True):
+          return self.different_type_children(entities)
+
+Once this is done, you can define your common view as follows::
+
+  class E1E2CommonView(baseviews.PrimaryView):
+      accepts = ('Entity11, 'Entity2')
+      
+      def render_entity_relations(self, entity, siderelations):
+          self.wview('list', entity.children(entities=False))
+