DISPLACEing macros and purified code.

[Alan Bawden <ALAN at MIT-MC>]

Is there any reason that the function DISPLACE couldn't check the
purity of its first argument and NOT displace if it is pure?? (Simply
return the second argument.)

I realize that this makes the contract of DISPLACE even more unusual
than it is (I seem to remember resistence to the PROGN hack for this
reason) but after all the whole point of DISPLACE is that it should be
the correct function to call to do "what you always want a displacing
macro to do"...