function mode( k ) {
	if ( isnan( k ) || k < 0.0 ) {
		return NaN;
	}
	return max( k - 2.0, 0.0 );
}
Returns the mode of a chi-squared distribution.

@param {NonNegativeNumber} k - degrees of freedom
@returns {NonNegativeNumber} mode

@example
var v = mode( 9.0 );
// returns 7.0

@example
var v = mode( 1.0 );
// returns 0.0

@example
var v = mode( -0.2 );
// returns NaN

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