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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 closed_issue(issue, after=None):
 """Returns True iff this issue was closed after given date. If after not given, only checks if issue is closed."""

    if issue['state'] == 'closed':
        if after is None or parse_timestamp(issue['closed_at']) > after:
            return True
    return False