      |  registered. Raises a #[code KeyError] otherwise.
  
  +aside-code("Example").
      from spacy.tokens import Doc
      Doc.set_extension('is_city', default=False)
      extension = Doc.get_extension('is_city')
      assert extension == (False, None, None, None)
  
  +table(["Name", "Type", "Description"])
      +row
          +cell #[code name]
 
</s> remove     Doc.set_extension('is_city', default=False)
    assert Doc.has_extension('is_city')
 </s> add     Doc.set_extension('has_city', default=False)
    assert Doc.has_extension('has_city')
    Doc.set_extension('has_city', default=False)
    extension = Doc.get_extension('has_city')
      |  registered. Raises a #[code KeyError] otherwise.
  
  +aside-code("Example").
      from spacy.tokens import Doc
    Doc.set_extension('has_city', default=False)
    extension = Doc.get_extension('has_city')
    Doc.set_extension('has_city', default=False)
    extension = Doc.get_extension('has_city')
      assert extension == (False, None, None, None)
  
  +table(["Name", "Type", "Description"])
      +row
          +cell #[code name]
keep keep keep keep replace replace keep keep keep keep keep
 Use better examples for Doc extension methods 
website/api/doc.jade