function median( lambda ) {
	if ( isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	return ( 1.0 / lambda ) * LN2;
}
Returns the median of an exponential distribution.

@param {NonNegativeNumber} lambda - rate parameter
@returns {NonNegativeNumber} median

@example
var v = median( 9.0 );
// returns ~0.077

@example
var v = median( 1.0 );
// returns ~0.693

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

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