code: def cns_vwl_pstr(self):
          
          return IPAString(ipa_chars=[c for c in self.ipa_chars if (c.is_letter) or (c.is_suprasegmental and c.is_primary_stress)])
docs: Return a new IPAString, containing only:
          
          1. the consonants,
          2. the vowels, and
          3. the primary stress diacritics
  
          in the current string.
  
          :rtype: IPAString