WHERE is the recaptchalib.php file?!

I'm probably being dim, but I can't see where the recaptchalib.php is?Surely this is needed as the code says:require_once('recaptchalib.php'); -- You received this message because you are subscribed to the Google Groups "reCAPTCHA" group. To unsubscribe from this group and stop receiving emails from it, send an email to recaptcha+unsubscribe@googlegroups.com. To post to this group, send email to recaptcha@googlegroups.com. Visit this group at http://groups.google.com/group/recaptcha. For more options, visit https://groups.google.com/groups/opt_out.