rurban/b-debug — pull request #1 by arc (2014-08-03 10:45:03+00:00)
Title: Avoid `exists` on an array element

The core has described this usage as "deprecated and likely to be removed"
for some time, and work is in progress in blead to start emitting a genuine
deprecation warning for such code.

In this case, using `defined` instead of `exists` seems to work fine.