[R6RS] Draft of arithmetic SRFI

[dyb at cs.indiana.edu]

> procedure: fx+ fx1 fx2
> procedure: fx- fx1 fx2
> procedure: fx* fx1 fx2

>     These procedures return the unique fixnum that is congruent mod hi-lo+1 to the sum, difference, or product of their arguments.

As I mentioned in Boston, I would prefer that these (and other arithmetic
procedures, where applicable) signal an error (raise an exception)
whenever the "normal" (non wrapping) arithmetic result cannot be
represented as a fixnum.  I don't think there's much use to the programmer
in wrapping when the size in bits isn't specified, and there is utility
in having an error signaled so that one is notified if overflow occurs.

Kent