R4RS 6.5.5  (- number1 ...)  ==>  number

With two or more arguments, this procedure returns the difference of
its arguments, associating to the left. With one argument, however,
it returns the additive inverse of its argument.

(- 3 4)    ==>  -1
(- 3 4 5)  ==>  -6
(- 3)      ==>  -3
