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

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

@example
var v = variance( 9.0 );
// returns ~0.012

@example
var v = variance( 1.0 );
// returns 1.0

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

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