function mgf( t, r, p ) {
	return addon( t, r, p );
}
Evaluates the moment-generating function (MGF) for a negative binomial distribution.

@private
@param {number} t - input value
@param {PositiveNumber} r - number of successes until experiment is stopped
@param {Probability} p - success probability
@returns {number} evaluated MGF

@example
var y = mgf( 0.05, 20.0, 0.8 );
// returns ~267.839

@example
var y = mgf( 0.1, 20.0, 0.1 );
// returns ~9.347

@example
var y = mgf( 0.5, 10.0, 0.4 );
// returns ~42822.023

@example
var y = mgf( 0.1, 0.0, 0.5 );
// returns NaN

@example
var y = mgf( NaN, 20.0, 0.5 );
// returns NaN