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

	/**
	* Evaluates the cumulative distribution function (CDF) for a Planck 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 -expm1( -lambda * ( floor(x)+1.0 ) );
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for a Planck distribution with shape parameter `lambda`.

@param {PositiveNumber} lambda - shape parameter
@returns {Function} CDF

@example
var cdf = factory( 1.5 );
var y = cdf( 3.0 );
// returns ~0.9975

y = cdf( 1.0 );
// returns ~0.9502