updated the documentation with the special values, and the idea of

column directories and column group directories.

doc/source/types.rst

@@ -6,20 +6,12 @@ OBITypes
 .. image:: ./UML/OBITypes_UML.png
 :download:`html version of the OBITypes UML file <UML/OBITypes_UML.class.violet.html>`
 
-.. note::
-    All OBITypes have an associated NA (Not Available) value.
-    We have currently two ideas for implementing NA values:
-    
-    		- By specifying an explicit NA value for each type
-    		- By adding to each column of an OBIDMS a bit vector
-    		  indicating if the value is defined or not.
-
-
 
 .. toctree::
    :maxdepth: 2
 
    The elementary types <elementary>
    The containers <containers>
+   Special values <specialvalues>