code: @Deprecated
    private Parser createLookaheadParser() {
      return new Parser(
          config,
          new LookaheadErrorReporter(),
          this.scanner.getFile(),
          this.scanner.getOffset(),
          inGeneratorContext());
    }
docs: Forks the parser at the current point and returns a new
  parser for speculative parsing.
  
  @deprecated Creating a lookahead parser often leads to exponential parse times
  (see issues #1049, #1115, and #1148 on github) so avoid using this if possible.