function cosm1( x ) {
	var x2;
	if ( x < -PIO4 || x > PIO4 ) {
		return cos( x ) - 1.0;
	}
	x2 = x * x;
	return ( -0.5*x2 ) + ( x2*x2*polyval( x2 ) );
}
Computes the cosine of a number minus one.

@param {number} x - input value (in radians)
@returns {number} cosine minus one

@example
var v = cosm1( 0.0 );
// returns 0.0

@example
var PI = require( '@stdlib/constants/float64/pi' );

var v = cosm1( PI/4.0 );
// returns ~-0.293

@example
var PI = require( '@stdlib/constants/float64/pi' );

var v = cosm1( -PI/6.0 );
// returns ~-0.134

@example
var v = cosm1( NaN );
// returns NaN