Emits the background knowledge in prd format.

def create_prd_file(self):
        '''
        Emits the background knowledge in prd format.
        '''
        prd_str = ''
        prd_str += '--INDIVIDUAL\n'
        prd_str += '%s 1 %s cwa\n' % (self.db.target_table, self.db.target_table)
        prd_str += '--STRUCTURAL\n'
        for ftable, ptable in self.db.reverse_fkeys.items():
            prd_str += '%s2%s 2 1:%s *:%s 1 cwa li\n' % (ptable, ftable[0], ptable, ftable[0])
        prd_str += '--PROPERTIES\n'
        prd_str += 'class 2 %s #class cwa\n' % self.db.target_table
        for table, cols in self.db.cols.items():
            for col in cols:
                if col != self.db.pkeys[table] and col not in self.db.fkeys[table] and (
                                table != self.db.target_table or col != self.db.target_att):
                    prd_str += '%s_%s 2 %s #%s_%s 1 cwa\n' % (table, col, table, table, col)
        return prd_str