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`atan2', `atan2f'--arc tangent of y/x

     #include <math.h>
     double atan2(double Y,double X);
     float atan2f(float Y,float X);
`atan2' computes the inverse tangent (arc tangent) of Y/X.  `atan2'
produces the correct result even for angles near pi/2 or -pi/2 (that
is, when X is near 0).
   `atan2f' is identical to `atan2', save that it takes and returns
`atan2' and `atan2f' return a value in radians, in the range of -pi to
   If both X and Y are 0.0, `atan2' causes a `DOMAIN' error.
   You can modify error handling for these functions using `matherr'.
`atan2' is ANSI C.  `atan2f' is an extension.