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

	/**
	* Evaluates the quantile function for a Bradford distribution.
	*
	* @private
	* @param {Probability} p - input value
	* @returns {number} evaluated quantile function
	*
	* @example
	* var y = quantile( 0.3 );
	* // returns <number>
	*/
	function quantile( p ) {
		if (
			isnan( p ) ||
			p < 0.0 ||
			p > 1.0
		) {
			return NaN;
		}
		return expm1( p * log1p( c ) ) / c;
	}
}
Returns a function for evaluating the quantile function for a Bradford distribution with shape parameter `c`.

@param {PositiveNumber} c - shape parameter
@returns {NonNegativeNumber} quantile function

@example
var quantile = factory( 5.0 );
var y = quantile( 0.4 );
// returns ~0.210

y = quantile( 0.8 );
// returns ~0.639

y = quantile( 1.0 );
// returns 1.0