[TODO: describe (only NRE-wiki page of relevance appears to be [L1 ]); rename the command to something more tclish?] Consequence of NRE.
Lars H: A thought: how is this different from what can be had through the age-old technique of creating a dummy local variable and putting an unset trace on it? Basic implementation:
proc atProcExit {script} {
upvar 1 atProcExitScripts arr
set arr($script) {}
trace add variable arr($script) unset "$script\n\#"
}