docs: def _tap(tap, runas=None):
      '''
      Add unofficial GitHub repos to the list of formulas that brew tracks,
      updates, and installs from.
      '''
      if tap in _list_taps():
          return True
  
      cmd = 'tap {0}'.format(tap)
      try:
          _call_brew(cmd)
      except CommandExecutionError:
          log.error('Failed to tap "%s"', tap)
          return False
  
      return True