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   atan2

   Usage
   #include <math.h>
   double atan2(double y, double x);

   Description
   atan2  returns  the arc tangent of y/x in the range  of  -p  to
   p. atan2 uses the signs of y and x to determine the quadrant of
   the return value.

   Return Value
   atan2 returns the arc tangent of the input values. If x and y are both
   0, atan2 sets errno to EDOM, prints a DOMAIN error message to  stderr,
   and  returns  0. Error handling can be modified through  the  function
   matherr.