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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 vizlib_unary_features(span):
 """ Visual-related features for a single span """

    if not span.sentence.is_visual():
        return

    for f in get_visual_aligned_lemmas(span):
        yield f"ALIGNED_{f}", DEF_VALUE

    for page in set(span.get_attrib_tokens("page")):
        yield f"PAGE_[{page}]", DEF_VALUE