function mean( r, p ) {
	return addon( r, p );
}
Returns the expected value of a negative binomial distribution.

@private
@param {PositiveNumber} r - number of failures until experiment is stopped
@param {Probability} p - success probability
@returns {NonNegativeNumber} expected value

@example
var v = mean( 100, 0.2 );
// returns 400.0

@example
var v = mean( 20, 0.5 );
// returns 20.0

@example
var v = mean( 10.3, 0.8 );
// returns ~2.575

@example
var v = mean( -2, 0.5 );
// returns NaN

@example
var v = mean( 20, 1.1 );
// returns NaN

@example
var v = mean( 20, NaN );
// returns NaN

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