Enqueue a LDAP modification.

        Arguments:
        dn -- the distinguished name of the object to modify
        mod -- an ldap modfication entry to enqueue

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)