reason behind the proposed change to setplist

[JONL at MIT-MC (Jon L White)]

The reason is quite obscure, but is purely a question of compiler
ease and efficiency.  Since it is so obscure, I talked it out with
GLS once, (and he agreed), but don't really want to do it again, 
unless there is large objection to the proposed change in return value.
In fact, some (PRATT is among them) think that the change to return
second arg is more consistent with other functions.