text: from IPython.core.magic import Magics, magics_class, cell_magic
  import os
  import sys
  
  _TAB = None
  _LOG = sys.stderr.write
  
  # the tab selected last time
  _LAST_TAB = None
  _LAST_TAB_FILE = '/tmp/d2lbook_last_selected_tab'
  if os.path.exists(_LAST_TAB_FILE):
      with open(_LAST_TAB_FILE) as f:
          _LAST_TAB = f.read().strip()
  
  def select_tab(tab=_LAST_TAB):
      _LOG(f'Selected tab "{tab}", all other code cells not marked as "{tab}" will be ignored in execution.\n')
      _LOG('This code block will be deleted during build.')
      sys.modules[__name__]._TAB = tab
      if tab:
          with open(_LAST_TAB_FILE, 'w') as f:
              f.write(tab+'\n')
  
  def interact_select(*tabs):
      if len(tabs) == 1 and isinstance(tabs[0], (list, tuple)):
          tabs = tabs[0]
      from ipywidgets import interact
      interact(select_tab, tab=list(tabs))
  
  def selected(*tabs):
      if len(tabs) == 1 and isinstance(tabs[0], (list, tuple)):
          tabs = tabs[0]
      return _TAB in tabs
  
  @magics_class
  class Tab(Magics):
      @cell_magic
      def tab(self, line, cell):
          tabs = [tab.strip() for tab in line.strip().split(',')]
          if _TAB in tabs or 'all' in tabs:
              self.shell.run_cell(cell)
          else:
              _LOG(f'Ignored to run as it is not marked as a "{_TAB}" cell.')
  
  
  def load_ipython_extension(ipython):
      ipython.run_cell('from d2lbook import tab')
      ipython.register_magics(Tab)
  