Purity and DISPLACE

[RWK at MIT-MC (Robert W. Kerns)]

In actual fact, I got disgusted enough to do something about it already.
MACROMEMO checks before clobbering, and does a hash-style memoizing instead
of a displace-style memoizing instead if the arg is pure.  For this purpose
this is superior to DISPLACE doing the checking and punting, since the macro
doesn't get re-expanded.  However, I agree that DISPLACE would be more
useful if it checked.  KMP, as you recall, would disagree.