function factory( lambda ) {
	var sigmaInv;
	var sigma;

	if ( isnan( lambda ) || lambda < 0.0 ) {
		return constantFunction( NaN );
	}
	if ( lambda === 0.0 ) {
		return degenerate( 0.0 );
	}
	sigma = sqrt( lambda );
	sigmaInv = 1.0 / sigma;
	return quantile;

	/**
	* Evaluates the quantile function for a Poisson distribution.
	*
	* @private
	* @param {Probability} p - input value
	* @returns {NonNegativeInteger} evaluated quantile function
	*
	* @example
	* var y = quantile( 0.3 );
	* // returns <number>
	*/
	function quantile( p ) {
		var guess;
		var corr;
		var x2;
		var x;

		if ( isnan( p ) || p < 0.0 || p > 1.0 ) {
			return NaN;
		}
		if ( p === 0.0 ) {
			return 0.0;
		}
		if ( p === 1.0 ) {
			return PINF;
		}
		// Cornish-Fisher expansion:
		if ( p < 0.5 ) {
			x = -erfcinv( 2.0 * p ) * SQRT2;
		} else {
			x = erfcinv( 2.0 * ( 1.0 - p ) ) * SQRT2;
		}
		x2 = x * x;

		// Skewness correction:
		corr = x + (sigmaInv * ( x2 - 1.0 ) / 6.0);
		guess = round( lambda + (sigma * corr) );
		return ( cdf( guess, lambda ) >= p ) ?
			search.left( guess, p, lambda ) :
			search.right( guess, p, lambda );
	}
}
Returns a function for evaluating the quantile function for a Poisson distribution with mean parameter `lambda`.

@param {NonNegativeNumber} lambda - mean parameter
@returns {Function} quantile function

@example
var quantile = factory( 5.0 );
var y = quantile( 0.4 );
// returns 4

y = quantile( 0.8 );
// returns 7

y = quantile( 1.0 );
// returns Infinity