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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def contrib_email(contrib_tag):
 """ Given a contrib tag, look for an email tag, and only return the value if it is not inside an aff tag """

    email = []
    for email_tag in extract_nodes(contrib_tag, "email"):
        if email_tag.parent.name != "aff":
            email.append(email_tag.text)
    return email if len(email) > 0 else None