function cdf( x ) {
		var xint;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 ) {
			return 0.0;
		}
		if ( x === PINF ) {
			return 1.0;
		}
		// Ensure left-continuity:
		xint = floor( x + 1e-7 );
		return betainc( p, r, xint + 1.0 );
	}
Evaluates the cumulative distribution function (CDF) for a negative binomial distribution.

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