-;;; there isn't really any successor, but if the end is an unknown,
-;;; non-tail call, then we emit an error trap just in case the
-;;; function really does return.
+;;; there isn't really any successor, but if the end is a non-tail
+;;; call to a function that's not *known* to never return, then we
+;;; emit an error trap just in case the function really does return.
+;;;
+;;; Trapping after known calls makes it easier to understand type
+;;; derivation bugs at runtime: they show up as nil-fun-returned-error,
+;;; rather than the execution of arbitrary code or error traps.