From the OpenGL Registry:
  "2012/08/13: specs named GL_ARB_debug_group, GL_ARB_debug_label, and
   GL_ARB_debug_output2 were published in error during the initial OpenGL 4.3
   release. All functionality in these documents was combined into
   the extension GL_KHR_debug. They have been withdrawn from the registry,
   and a few other extensions were renumbered to avoid holes in the numbering
   scheme."