function variance( p ) {
	if (
		isnan( p ) ||
		p <= 0.0 ||
		p >= 1.0
	) {
		return NaN;
	}
	return ( 1.0-p ) / ( p*p );
}
Returns the variance of a geometric distribution.

@param {Probability} p - success probability
@returns {PositiveNumber} variance

@example
var v = variance( 0.1 );
// returns ~90.0

@example
var v = variance( 0.5 );
// returns 2.0

@example
var v = variance( 1.1 );
// returns NaN

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