old_contents: 
  # Whitelist of environment variables for the subprocess to inherit
  # c.Spawner.env_keep = ['PATH', 'PYTHONPATH', 'CONDA_ROOT', 'CONDA_DEFAULT_ENV', 'VIRTUAL_ENV', 'LANG', 'LC_ALL']
  c.Spawner.env_keep = [ 'PATH', 'LD_LIBRARY_PATH', 'JAVA_HOME', 'CPATH', 'CMAKE_ROOT' ]
  
  # use jupyterlab as default user interface
  c.Spawner.default_url = '/lab'
  
  # set of usernames of admin users
  #
  # If unspecified, only the user that launches the server will be admin.
  #c.Authenticator.admin_users = set(['admin'])
  
  c.JupyterHub.extra_log_file = '/var/log/jupyterhub.log'
  
  # https://github.com/jupyterhub/jupyterlab-hub
  c.Spawner.cmd = ['jupyter-labhub']
new_contents: 
  # Whitelist of environment variables for the subprocess to inherit
  # c.Spawner.env_keep = ['PATH', 'PYTHONPATH', 'CONDA_ROOT', 'CONDA_DEFAULT_ENV', 'VIRTUAL_ENV', 'LANG', 'LC_ALL']
  c.Spawner.env_keep = [ 'PATH', 'LD_LIBRARY_PATH', 'JAVA_HOME', 'CPATH', 'CMAKE_ROOT' ]
  
  # use jupyterlab as default user interface
  #c.Spawner.default_url = '/lab'
  
  # set of usernames of admin users
  #
  # If unspecified, only the user that launches the server will be admin.
  #c.Authenticator.admin_users = set(['admin'])
  
  c.JupyterHub.extra_log_file = '/var/log/jupyterhub.log'
  
  # https://github.com/jupyterhub/jupyterlab-hub
  c.Spawner.cmd = ['jupyter-labhub']
subject: revert to regular jupyter notebook as default user interface.