code: def _camelcase_to_underscore(url):
      
      def upper2underscore(text):
          for char in text:
              if char.islower():
                  yield char
              else:
                  yield '_'
                  if char.isalpha():
                      yield char.lower()
      return ''.join(upper2underscore(url))
docs: Translate camelCase into underscore format.
  
      >>> _camelcase_to_underscore('minutesBetweenSummaries')
      'minutes_between_summaries'