classThis class is an implementation of['a node #extension as 'a]comment_impl :'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).
