Update bracket-push example

[sshine]

Avoids conversion to list. Terminates early on bad input.

[sshine]

Yes, you did.

I don't like to change reference solutions unless there's a clear benefit.

I like the implied policy of having examples rather than "best" solutions.

This policy reduces tension between contributors getting "their" solution in.

There's plenty of room for that on the "community solutions" page.