function variance( p ) {
	return addon( p );
}
Returns the variance of a Bernoulli distribution.

@private
@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