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

With two or more arguments, these procedures return the difference
or quotient of their arguments, associating to the left. With one
argument, however, they return the additive or multiplicative inverse
of their argument.

(- 3 4)    ==>  -1
(- 3 4 5)  ==>  -6
(- 3)      ==>  -3
(/ 3 4 5)  ==>  0.15
(/ 4)      ==>  0.25

