'invoke PDB set_trace debugging, dropping any IO capturing.'

@classmethod
def set_trace(cls):

    import _pytest.config
    frame = sys._getframe().f_back
    if (cls._pluginmanager is not None):
        capman = cls._pluginmanager.getplugin('capturemanager')
        if capman:
            capman.suspendcapture(in_=True)
        tw = _pytest.config.create_terminal_writer(cls._config)
        tw.line()
        tw.sep('>', 'PDB    set_trace    (IO-capturing    turned    off)')
        cls._pluginmanager.hook.pytest_enter_pdb(config=cls._config)
    cls._pdb_cls().set_trace(frame)
