classThis class is an implementation of[< clone : 'a; node : 'a node;comment_impl :
set_node : 'a node -> unit; .. >
as 'a]'a ->['a]node
node which
   realizes comment nodes. You can create a new exemplar by
   let exemplar = new comment_impl ext_obj
   which creates an empty comment which already contains a
   reference to the extension ext_obj, but is otherwise empty.
   Use the method Pxp_document.node.create_other on the exemplar 
   to get a filled copy
   of the examplar (i.e. we need two steps to create nodes).