substitution group elements

In section 4.6(substitution group) of XML Schema Part 0 :Primer we have two element declarations "CustomerComment" and "ShipComment" which are assigned to a substitution group whose head element is "comment".With these declarations CustomerComment and ShipComment elements can be substituted for comment.
 I would like to know whether with this declaration alone, is the reverse possible. that is, can the element comment be substituted in the place of the other two elements. OR should I need to declare comment as substitutable for these elements.
 
with Regards,
V.Gayathri.
 
 
  

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