On 06/12/2017 01:44 PM, John Cowan wrote:
>     flatan arctan(y/x) is not always equivalent to atan2(y,x), so
>     something is wrong with this spec.
>
>
> Unfortunately, the wording of Posix is plain:  "Upon successful
> completion, these functions shall return the arc tangent of y/ x
> in the range [π-,π] radians."  Various special cases are then
> stated, for which see
> <http://pubs.opengroup.org/onlinepubs/9699919799/functions/atan2.html>.

  (flatan (fl/ y x)) returns values only between $-\pi/2$ and $\pi/2$,
while (flatan x y) returns values between $-\pi$ and $\pi$, which is
what I think Will was referring to.

Perhaps one could say:

Upon successful completion, these functions shall return the arc tangent
of y/ x in the range [π-,π] radians, using the signs of y and x to
choose the correct quadrant for the result.

Brad