docs: def to_url(self, site='amazon', country='us'):
          """Generate a link to an online book site.
  
          Args:
              site (str): Site to create link to
              country (str): Country specific version of ``site``
  
          Returns:
              ``str``: URL on ``site`` for book
  
          Raises:
              SiteError: Unknown site value
              CountryError: Unknown country value
  
          """
          try:
              try:
                  url, tlds = URL_MAP[site]
              except ValueError:
                  tlds = None
                  url = URL_MAP[site]
          except KeyError:
              raise SiteError(site)
          inject = {'isbn': self._isbn}
          if tlds:
              if country not in tlds:
                  raise CountryError(country)
              tld = tlds[country]
              if not tld:
                  tld = country
              inject['tld'] = tld
          return url % inject