You can try that, but it produces the wrong answer: "where" is still zero-based. You would also need 1+ i.e.

    q)1+where mod[;2] sum 0=(1_til 500) mod/: til 500
    1 4 9 16 25 36 49 64 81 100 121 144 169 196 225 256 289 324 361 400 441 484

...but this is not a huge bother for me: 0 mod 0 is 0N which doesn't equal zero, and I think the extra 1_ or 1+ distracts somewhat from the point I'm trying to make.