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

	/**
	* Evaluates the natural logarithm of the cumulative distribution function (CDF) for an exponential distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logCDF
	*
	* @example
	* var y = logcdf( 2.0 );
	* // returns <number>
	*/
	function logcdf( x ) {
		if ( x < 0.0 ) {
			return NINF;
		}
		return log1p( -exp( -lambda * x ) );
	}
}
Returns a function for evaluating the natural logarithm of the cumulative distribution function (CDF) for an exponential distribution with rate parameter `lambda`.

@param {PositiveNumber} lambda - rate parameter
@returns {Function} logCDF

@example
var logcdf = factory( 0.5 );
var y = logcdf( 3.0 );
// returns ~-0.252

y = logcdf( 1.0 );
// returns ~-0.933