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`acos', `acosf'--arc cosine

     #include <math.h>
     double acos(double X);
     float acosf(float X);
`acos' computes the inverse cosine (arc cosine) of the input value.
Arguments to `acos' must be in the range -1 to 1.
   `acosf' is identical to `acos', except that it performs its
calculations on `floats'.
`acos' and `acosf' return values in radians, in the range of 0 to pi.
If X is not between -1 and 1, the returned value is NaN (not a number)
the global variable `errno' is set to `EDOM', and a `DOMAIN error'
message is sent as standard error output.
   You can modify error handling for these functions using `matherr'.