The last logical operator available on the 83 series takes only one value as input. not( comes with its own parentheses to make up for this loss. Quite simply, it negates the input: False becomes True (1) and True returns False (0). not( can be nested; one use is to make any True value into a 1.
:not(0) 1 :not(-20 and 14) 0 :not(not(2)) 1
not(not(X)) will make any value X into 1 if it's not 0, and will keep it 0 if it is.
not(X) and X=0 have the same truth value, but not( is shorter if the closing parenthesis is omitted:
:If A=0 can be :If not(A