<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_grant(self, grant):
 """Add a grant to this AccessToken"""

        if not isinstance(grant, AccessTokenGrant):
            raise ValueError('Grant must be an instance of AccessTokenGrant.')
        self.grants.append(grant)