It's not for clarity, it's to prevent plopping an EXIT control into the ENTRY
slot and vice versa. I have a hell of a time trying to visually differentiate
those, and a buggy pair isn't guaranteed to be detected at runtime, e.g. if both
are swapped, all bets are off, and if one is duplicated, odds the warn may or may
not show up unless hardware actually supports at least one of the controls, if not
both.