indexes as ordered set - issue in code

[maxvolpi]

I am trying to reproduce the code on page 107 of the first edition (and it is the same on Github).
When I do indA & indB I get a strange result: [0,3,5, 7,9] while if I use the code set(indA) & set(indB) I get the expected result
{3,5,7}. I also get the same correct result if I use the method indA.intersection(indB).

Likewise, I get funny results with | and ^. while the equivalent set operations and the methods all work.

It is not clear why this is happening, as I have tried it from scratch a few times, to avoid typos