code: function /* 6.2.2 */$$core$$IsStructurallyValidLanguageTag(locale) {
          // represents a well-formed BCP 47 language tag as specified in RFC 5646
          if (!$$exp$$expBCP47Syntax.test(locale))
              return false;
  
          // does not include duplicate variant subtags, and
          if ($$exp$$expVariantDupes.test(locale))
              return false;
  
          // does not include duplicate singleton subtags.
          if ($$exp$$expSingletonDupes.test(locale))
              return false;
  
          return true;
      }
docs: Sect 6.2 Language Tags ====================== 
  The IsStructurallyValidLanguageTag abstract operation verifies that the locale
  argument (which must be a String value)
  
  - represents a well-formed BCP 47 language tag as specified in RFC 5646 section
  2.1, or successor,
  - does not include duplicate variant subtags, and
  - does not include duplicate singleton subtags.
  
  The abstract operation returns true if locale can be generated from the ABNF
  grammar in section 2.1 of the RFC, starting with Language-Tag, and does not
  contain duplicate variant or singleton subtags (other than as a private use
  subtag). It returns false otherwise. Terminal value characters in the grammar are
  interpreted as the Unicode equivalents of the ASCII octet values given.