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$DECR
: Decrement a macro$DECR(N)
redefines the numeric macro N to be one less
than its previous value. (If N does not simplify to a number, an
error results.) In other words, in the language of C the effect is to
say N--
.
The two-argument form $DECR(N,m)
executes the equivalent of
N -= m
.