code: def ssml_sub(self, words, alias=None, **kwargs):
          
          return self.nest(SsmlSub(words, alias=alias, **kwargs))
docs: Create a <Sub> element
  
          :param words: Words to be substituted
          :param alias: Substitute a different word (or pronunciation) for selected text such as an acronym or abbreviation
          :param kwargs: additional attributes
  
          :returns: <Sub> element