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From:     GSB@MIT-ML
Date: Sat, 8 Apr 79 07:55:29 GMT
Original-Date: 08/04/79 03:55:29 EDT
    If this gets implemented, it probably would be a good idea to
    allow differentiation of unbound variables which have the shared
    value cell and those that do not (the way (get var 'value) returns
    NIL if var has never been bound, else (NIL . UNBOUND)).  Maybe 
    -1 in one case and 0 in the other, making PLUSP the canonical
    check on VALUE-CELL-LOCATION's result to see if the var is bound?
I meant to say in my original proposal, that 0 was to be returned
if there were no "private" value cell, and the address of the "private"
value cell would be returned when there was one, regardless of whether
or not the symbol was "bound".  A combination of BOUNDP and 
VALUE-CELL-LOCATION can distinguish the two cases you mention,
namely, "unbound, but no private value cell", and "unbound, with
unoubnd marker in private value cell".