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

	/**
	* Evaluates the cumulative distribution function (CDF) for a Bradford distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*
	* @example
	* var y = cdf( 0.5 );
	* // returns <number>
	*/
	function cdf( x ) {
		if (
			isnan( x )
		) {
			return NaN;
		}
		if ( x <= 0.0 ) {
			return 0.0;
		}
		if ( x >= 1.0 ) {
			return 1.0;
		}
		return log1p( c * x ) / log1p( c );
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for a Bradford distribution with shape parameter `c`.

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

@example
var cdf = factory( 5.0 );
var y = cdf( 0.5 );
// returns ~0.699

y = cdf( 1.0 );
// returns 1.0