docs: def set_trace(a_frame=None):
      """ Breaks on the line that invoked this function or at given frame.
      User can then resume execution.
      
      To call set_trace() use:
      
      .. code-block:: python
  
          import ikp3db ; ikp3db.set_trace()
  
      :param a_frame: The frame at which to break on.
      :type a_frame: frame
      
      :return: An error message or None is everything went fine.
      :rtype: str or None
  
      """
      if not ikpdb:
          return "Error: IKP3db must be launched before calling ikpd.set_trace()."
  
      if a_frame is None:
          a_frame = sys._getframe().f_back
      ikpdb._line_tracer(a_frame)
      return None