function variance( lambda ) {
	if ( isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	return lambda;
}
Returns the variance of a Poisson distribution.

@param {NonNegativeNumber} lambda - mean parameter
@returns {NonNegativeNumber} variance

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

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

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

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