在看Flask-Web开发中，关于9.3章角色验证这里有一个问题：
class User(UserMixin,db.Model):

__tablename__ = 'users'
id = db.Column(db.Integer,primary_key=True)
email = db.Column(db.String(64),unique=True,index=True)
username=db.Column(db.String(64),unique=True,index=True,)
role_id = db.Column(db.Integer,db.ForeignKey('roles.id'))
password_hash = db.Column(db.String(128))
confirmed = db.Column(db.Boolean,default=False)

def __init__(self,**kwargs):
    super(User,self).__init__(**kwargs)
    if self.role is None:
        if self.email==current_app.config['FLASKY_ADMIN']:
            self.role=Role.query.filter_by(permissions=0xff).first()
        if self.role is None:
            self.role=Role.query.filter_by(default=True).first()

def can(self.permissions):
    return self.role is not None and \
        (self.role.permissions & permissions) == permissions

就是这个can函数，为什么这里要用位与运算来验证(self.role.permissions & permissions) == permissions，直接用 self.role.permissions == permissions可以么