\directlua0{
local function traverse(h)
  for n in node.traverse (h)
    do texio.write_nl('node id='..node.type(n.id))
    if n.id == node.id('hlist') then
      traverse(n.list)
    end
  end
end
callback.register('pre_linebreak_filter',
  function(h)
    traverse(h)
    return true
  end
)
}

\hbox{Not seen}

Hello \hbox{seen}
\bye
