function factory( lambda ) {
	if ( isnan( lambda ) || lambda < 0.0 ) {
		return constantFunction( NaN );
	}
	if ( lambda === 0.0 ) {
		return degenerate( 0.0 );
	}
	return cdf;

	/**
	* Evaluates the cumulative distribution function (CDF) for a Poisson distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*
	* @example
	* var y = cdf( 2.0 );
	* // returns <number>
	*/
	function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 ) {
			return 0.0;
		}
		if ( x === PINF ) {
			return 1.0;
		}
		return gammainc( lambda, floor( x ) + 1.0, true, true );
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for a Poisson distribution with mean parameter `lambda`.

@param {NonNegativeNumber} lambda - mean parameter
@returns {Function} CDF

@example
var cdf = factory( 5.0 );
var y = cdf( 3.0 );
// returns ~0.265

y = cdf( 8.0 );
// returns ~0.932