Oh, good question. From the [Bash changelog](http://tiswww.case.edu/php/chet/bash/CHANGES) it looks to me like it might have been added in 3.1. Is that old enough? [Maybe](http://tiswww.case.edu/php/chet/bash/bashtop.html#Availability).

Sorry about these mostly unrelated changes. I was trying to figure out why the space in `-Duser.dir=` was causing problems, and I was hoping these would help, but they did not. (I'm not very good with Bash.) In the end the fix was c132333a3a11ee329c0dd2a3537713f3c1ea15b0 — your change, that I did not have.

Let me know if you would prefer these `+=` diffs reverted!