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

@private
@param {NonNegativeNumber} lambda - rate parameter
@returns {NonNegativeNumber} mode

@example
var v = mode( 9.0 );
// returns 0.0

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

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

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