<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def charge(self, user, vault_id=None):
 """If vault_id is not passed this will assume that there is only one instane of user and vault_id in the db."""

        assert self.is_in_vault(user)
        if vault_id:
            user_vault = self.get(user=user, vault_id=vault_id)
        else:
            user_vault = self.get(user=user)