function median( lambda ) {
	return addon( lambda );
}
Returns the median of an exponential distribution.

@private
@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