DISPLACE

[KMP at MIT-MC (Kent M. Pitman)]

I recognize that there exists code which will be screwed by not checking
purity and that there may also exist code which will be screwed up by the
check. ALAN: are you doing explicit displace calls or calling defmacro
with displace optin requested? 
Is the functin REPLACE in use all over? Probably, huh? That would be a fine 
name for the functin I want. I would still prefer
that operators which operate in a code-oriented manner rather than
a list-oriented manner be named to identify that. Eg, a code-subst functin
would be more usful than a subst functin like we have right now but throwing
out the simple semantics for the simple operator is unfair. .. i don't know.
many hard decisions. i guess we should do something that will move toward 
compatibility with lispm. how heavily is displace used on lispm. ..?
can we postpone doing anything drastic until i return to a terminal where i
can compose my thoughts reasonably ? tnx