<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def count_of_certain_category(cat_id, tag=''):
        '''
        Get the count of certain category.
        '''

        if cat_id.endswith('00'):
            # The first level category, using the code bellow.
            cat_con = TabPost2Tag.par_id == cat_id
        else:
            cat_con = TabPost2Tag.tag_id == cat_id

        if tag:
            condition = {
                'def_tag_arr': [tag]
            }
            recs = TabPost2Tag.select().join(
                TabPost,
                on=((TabPost2Tag.post_id == TabPost.uid) & (TabPost.valid == 1))
            ).where(
                cat_con & TabPost.extinfo.contains(condition)
            )
        else:
            recs = TabPost2Tag.select().where(
                cat_con
            )

        return recs.count()