function median( lambda ) {
	if ( isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	if ( lambda === 0.0 ) {
		return 0.0;
	}
	return floor( lambda + ( 1.0/3.0 ) - ( 0.02/lambda ) );
}
Returns the median of a Poisson distribution.

@param {NonNegativeNumber} lambda - mean parameter
@returns {NonNegativeInteger} median

@example
var v = median( 9.0 );
// returns 9

@example
var v = median( 1.0 );
// returns 1

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

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