function variance( lambda ) {
	var temp;
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	temp = expm1( -lambda );
	return exp( -lambda ) / ( temp * temp );
}
Returns the variance of a Planck distribution.

@param {PositiveNumber} lambda - shape parameter
@returns {PositiveNumber} variance

@example
var v = variance( 0.1 );
// returns ~99.9167

@example
var v = variance( 1.5 );
// returns ~0.3697

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

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