How to use the emmett.orm.helpers.Callback function in emmett

def before_delete(f):
    return Callback(f, '_before_delete')

def __init__(self, f, t):
        self.t = []
        if isinstance(f, Callback):
            self.t += f.t
            f = f.f
        self.f = f
        self.t.append(t)
        self._inst_count_ = Callback._inst_count_
        Callback._inst_count_ += 1

def after_delete(f):
    return Callback(f, '_after_delete')

def before_update(f):
    return Callback(f, '_before_update')

def after_update(f):
    return Callback(f, '_after_update')

def after_insert(f):
    return Callback(f, '_after_insert')

def before_insert(f):
    return Callback(f, '_before_insert')

def __init__(self, f, t):
        self.t = []
        if isinstance(f, Callback):
            self.t += f.t
            f = f.f
        self.f = f
        self.t.append(t)
        self._inst_count_ = Callback._inst_count_
        Callback._inst_count_ += 1