def raise(exception_klass=Parslet::ParseFailed)
      exception = exception_klass.new(self.to_s, self)
      Kernel.raise exception
    end
def raise(exception_klass=Parslet::ParseFailed)
      exception = exception_klass.new(self.to_s, self)
      Kernel.raise exception
    end
Signals to the outside that the parse has failed. Use this in
 conjunction with .format for nice error messages.