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     The acos, asin, atan, and atan2 functions return the arccosine,
     the arcsine, and the arctangent values of <x>, respectively. The
     argument <x> is specified in radians. The atan2 function uses the
     signs of both arguments to determine the quadrant of the return
     value. The functions, argument ranges, and return value ranges are
     listed below:
 
     Function     Argument Range     Return Value Range
 
     acos         -1 to 1            0 to π
     asin         -1 to 1            -π/2 to π/2
     atan         No limit           -π/2 to π/2
     atan2        No limit           -π to π
 
     The long double functions use an 80-bit long double form of
     arguments and return values. In all other respects, they are
     identical to the regular functions.
 
     For the acos and asin functions, <x> must be between -1 and 1. If
     <x> is less than -1 or greater than 1, acos and asin set errno to
     EDOM, print a DOMAIN error message to stderr, and return 0.
 
     If both arguments to atan2 are 0, the function sets errno to EDOM,
     prints a DOMAIN error message to stderr, and returns 0.
 
     Error handling can be modified with the matherr (or _matherrl)
     routine.
 
     Return Value
 
     The return values for these functions are described above.
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