code: def vizlib_unary_features(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
docs: Visual-related features for a single span