function logpmf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( !isNonNegativeInteger( x ) ) {
			return NINF;
		}
		return ln( p ) - ln( r + x ) + ln( ibetaDerivative( p, r, x + 1.0 ) );
	}
Evaluates the natural logarithm of the probability mass function (PMF) for a negative binomial distribution.

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

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