tutorial: make it work with the default lexer (parse integers)
tutorial: make it work with the default lexer (parse integers)

Closes: #231

Signed-off-by: Marco Molteni <d3ec57bc6a89ff8a13b81c0a963aacb59a57256a@mailbox.org>

alecthomas/participle,alecthomas/participle,alecthomas/participle