function mgf( t ) {
		if ( t >= -ln( p ) ) {
			return NaN;
		}
		return pow( ( (1.0 - p) * exp( t ) ) / ( 1.0 - (p * exp( t )) ), r );
	}
Evaluates the moment-generating function (MGF) for a negative binomial distribution.

@private
@param {number} t - input value
@returns {number} evaluated MGF

@example
var y = mgf( 0.5 );
// returns <number>