Hi Yogi,
This used to be a valid expression:
fexpr~$s3[$x1[0]] = $x2[0]; $s3[$x1[0]+22050] = $x2[0]
Thanks for sending this bug report on expr. You are correct that this should work and I have identified the fix for it which I will push to repo in a day or two. The bug was introduced with my major changes to support strings in expr. Please stay tuned.
Best, Shahrokh