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Re: Maclisp compatibility

"Lisp 1.5 should get no special treatment here: all of its important features
show up in Maclisp, and the ones that have changed or dropped away have done
so for good reason."

I am curious about one feature of Lisp 1.5 (and also Standard Lisp) which was
dropped from Maclisp.  I am referring to the Flag/FlagP property list functions.
I realize that Put(Symbol, Indicator, T) can serve the same function, but I
can't see any good reason why the others should have been dropped.  In an
obvious implementation of property lists Put/Get can use dotted pairs and
Flag/FlagP use atoms, making the property list itself sort of a corrupted
association list.  Maclisp and its descendants seem to use a flat list of
alternating indicators and values.  It isn't clear to me what advantage this
representation gives over the a-list.  Were Flag and FlagP dropped as a
streamlining effort, or what?