function pmf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( !isNonNegativeInteger( x ) ) {
			return 0.0;
		}
		return ( p / ( r + x ) ) * ibetaDerivative( p, r, x + 1.0 );
	}
Evaluates the probability mass function (PMF) for a negative binomial distribution.

@private
@param {number} x - input value
@returns {Probability} evaluated PMF

@example
var y = pmf( 2.0 );
// returns <number>