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`asin', `asinf'--arc sine

     #include <math.h>
     double asin(double X);
     float asinf(float X);
`asin' computes the inverse sine (arc sine) of the argument X.
Arguments to `asin' must be in the range -1 to 1.
   `asinf' is identical to `asin', other than taking and returning
   You can modify error handling for these routines using `matherr'.
`asin' returns values in radians, in the range of -pi/2 to pi/2.
If X is not in the range -1 to 1, `asin' and `asinf' return NaN (not a
number), set the global variable `errno' to `EDOM', and issue a `DOMAIN
error' message.
   You can change this error treatment using `matherr'.