The rule used was the intervening phrase. Intervening phrase or interrupter is used to modify the subject in a sentence but it still doesn't affect the number of the subject. Usually intervening phrase starts with in addition to, accompanied by, etc. Example: Kim, as well as the bridesmaids, help in designing the wedding dress. 

See that? The subject is Kim, and the verb used was help even though the bridesmaids was plural and it's part of the sentence. It still doesn't affect the number of subject. I repeat, intervening phrase doesn't affect the number of subject, it just modifies the subject. So yeah, I hope this helped :)