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Re: Shifted-0, Modular arithmetic

Of course, part of the complaint is that the PDP-10 instruction
set doesn't even implement the ring you described, but only
an approximation to it.  If it were a true ring then indeed
(* x 2) would be the same as (+ x x).  However, the PDP-10
IMUL instruction does not store the 36 low-order bits,
but the 35 low-order bits plus the correct sign.  That's why
(* 2 2_34.) => 0 instead of -2_35.