(dec! place ..args)
  place: a place
  args: zero or more forms
M

Performs in-place subtraction or decrement.

(dec! x 5 6) is equivalent to (= x (- x 5 6)), except that x is only evaluated once.

With only one argument, (dec! x) is equivalent to (dec! x 1).

See also: -

inc!
mul!