<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def enqueue_mod(self, dn, mod):
 """Enqueue a LDAP modification. Arguments: dn -- the distinguished name of the object to modify mod -- an ldap modfication entry to enqueue """

        # mark for update
        if dn not in self.__pending_mod_dn__:
            self.__pending_mod_dn__.append(dn)
            self.__mod_queue__[dn] = []

        self.__mod_queue__[dn].append(mod)