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

@private
@param {Probability} p - success probability
@returns {PositiveNumber} expected value

@example
var v = mean( 0.1 );
// returns 0.1

@example
var v = mean( 0.5 );
// returns 0.5

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

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