docs: public static MatchablePathElement buildMatchablePathElement(String rawJsonKey) {
          PathElement pe = PathElementBuilder.parseSingleKeyLHS( rawJsonKey );
  
          if ( ! ( pe instanceof MatchablePathElement ) ) {
              throw new SpecException( "Spec LHS key=" + rawJsonKey + " is not a valid LHS key." );
          }
  
          return (MatchablePathElement) pe;
      }