(Theres's a note on implement modules that says to do this as a module.)
Captcha cardtype - name is challenge, content is response
If "|cardtype|+Captcha" (Toggle) exists and is 1, when Anonymous tries to edit/rename/cardtype change/delete/comment on cards of that type, then make there a captcha.
In English:
"Basic+*captcha" exists but is 0 (and is 'inherited' by other cardtype)
"Account Request+*captcha" exists and is 1
The Captcha card itself lists cards with "*captcha" as their right part in close view.