Here's why:
* Predicate Nominative: A predicate nominative is a noun or pronoun that follows a linking verb (like "be," "become," "seem," "appear") and renames the subject. In this sentence, the linking verb is "has been".
* Subject: The subject is "Sherry".
* Complements: The words in parentheses, "(cook) (waitress) and (receptionist)," are all nouns that rename the subject "Sherry". They tell us what Sherry has been.
While the sentence is not grammatically perfect, it is clear that the words in parentheses are intended to function as predicate nominatives, even though they are not formally introduced with articles like "a" or "the."