... the ... doco show the same  (??{ code }) example for matching  (...) ... a  (?PARNO) solution ...

I'm interested in  (?PARNO) and  (?R) because the documentation examples for nested expressions are almost the same. (Indeed, the doc sez "Similar to "(??{ code })"..." re: (?PARNO).) The impression the docs give is that something like  (?R) is somehow 'safer'. This may be tied up with the fact that  (??{ code }) used to be broken for use with lexicals – a problem I believe was fixed somewhere around 5.16 or 5.18 (I only go as high as 5.14 right now).

Thanks to the 5.18 feedback on  (DEFINE). Yes, some variations work, some don't, and I don't understand the reason(s) for the differences.