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

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

@example
var v = variance( 0.1 );
// returns ~0.09

@example
var v = variance( 0.5 );
// returns 0.25

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

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