docs: def _add_eutils_api_key(url):
      """Adds eutils api key to the query
  
      :param url: eutils url with a query string
      :return: url with api_key parameter set to the value of environment
      variable 'NCBI_API_KEY' if available
      """
      apikey = os.environ.get("NCBI_API_KEY")
      if apikey:
          url += "&api_key={apikey}".format(apikey=apikey)
      return url