[_def]

> This matches the output format of Bash's builtin set -x tracing, but gives the script author more granular control of what is printed.

I get and love the idea but I'd consider this implementation an anti-pattern. If the output mimics set -x but isn't doing what that is doing, it can mislead users of the script.

[delusional]

Even worse, it mimics it poorly, hardcoding the PS4 to the default.

The author could also consider trapping debug to maybe be selective while also making it a little more automatic.