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`atan', `atanf'--arc tangent

     #include <math.h>
     double atan(double X);
     float atanf(float X);
`atan' computes the inverse tangent (arc tangent) of the input value.
   `atanf' is identical to `atan', save that it operates on `floats'.
`atan' returns a value in radians, in the range of -pi/2 to pi/2.
`atan' is ANSI C.  `atanf' is an extension.