Merge pull request #10 from delallea/nouiz-doc

doc/extending/ctype.txt

@@ -486,12 +486,20 @@ Output Guard
 ============
 
 We have an internal Op called OutputGuard. It is used for some
-verification of inplace/view Ops. It has a C implementation that
-increments/decrements Python reference counts. If your type represents a
-Python object, OutputGuard C code should work with your type. You must
-tell OutputGuard to generate C code when working with your
-type, as otherwise it will use Python code. You can achieve this with:
+verification of inplace/view Ops. Its C implementation increments and
+decrements Python reference counts, and thus only works with Python
+objects. If your new type represents Python objects, you should tell
+OutputGuard to generate C code when working with this type, as
+otherwise it will use Python code instead. This is achieved by
+calling:
 
 .. code-block:: python
 
    theano.compile.mode.register_OutputGuard_c_code(YOUR_TYPE_CLASS)
+
+Note that you should not call ``register_OutputGuard_c_code`` with your
+new type if this type is not associated to Python objects in the C code,
+as otherwise the reference count mechanism will crash. For instance,
+the ``Double`` type described here is associated to ``double`` objects
+in the C code, and consequently this type should not be registered in
+OutputGuard.