<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def map_db(self, plain_src_db):
 """Given a plain source database, return the list of target databases. Individual collections in a database can be mapped to different target databases, so map_db can return multiple databases. This function must return all target database names so we make the following restrictions on wildcards: 1) A wildcard appearing in the source database name must also appear in the target database name, eg "db*.col" => "new_db_*.new_col". 2) A wildcard appearing in the source collection name must also appear in the target collection name, eg "db.col*" => "new_db.new_col*". This is used by the CommandHelper for the dropDatabase command. """

        if not self._regex_map and not self._plain:
            return [plain_src_db]
        # Lookup this namespace to seed the plain_db dictionary
        self.lookup(plain_src_db + ".$cmd")
        return list(self._plain_db.get(plain_src_db, set()))