function shift_tab (tab)
{
  document.getElementById('tab_1').className = 'hidden';
  document.getElementById('tab_2').className = 'hidden';
  document.getElementById('tab_' + tab).className = 'visible';

  document.getElementById('pm_1').className = 'hidden';
  document.getElementById('pm_2').className = 'hidden';
  document.getElementById('pm_' + tab).className = 'visible';
}