`**` operator (or something else) for `math.pow`

[eitsupi]

See #3909

It would be useful to have an operator for powers, but although ** was about to be implemented in #3909, it did not work out and has not been implemented so far.

[max-sixty]

Yes!

This was an issue with doing right associativity. I discussed it on a call with @aljazerzen and he explained a way of doing it.

He also pushed a branch — https://github.com/PRQL/prql/tree/right-asoc — so I need to try and land that.