Class Pxp_document.comment_impl

class [< clone : 'a; node : 'a node;
       set_node : 'a node -> unit; .. >
     as 'a] comment_impl : 'a -> ['a] node

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).