function quantile( k ) {
		var guess;
		var corr;
		var x2;
		var x;

		if ( isnan( k ) || k < 0.0 || k > 1.0 ) {
			return NaN;
		}
		if ( k === 0.0 ) {
			return 0.0;
		}
		if ( k === 1.0 ) {
			return PINF;
		}

		// Cornish-Fisher expansion:
		if ( k < 0.5 ) {
			x = -erfcinv( 2.0 * k ) * SQRT2;
		} else {
			x = erfcinv( 2.0 * (1.0-k) ) * SQRT2;
		}
		x2 = x * x;

		// Skewness correction:
		corr = x + (sigmaInv * ( x2 - 1.0 ) / 6.0);
		guess = round( mu + (sigma * corr) );
		return ( cdf( guess, r, p ) >= k ) ?
			search.left( guess, k, r, p ) :
			search.right( guess, k, r, p );
	}
Evaluates the quantile function for a negative binomial distribution.

@private
@param {Probability} k - input value
@returns {NonNegativeInteger} evaluated quantile function

@example
var y = quantile( 0.3 );
// returns <number>