function cdf( x, c ) {
	if (
		isnan( c ) ||
		isnan( x ) ||
		c <= 0.0
	) {
		return NaN;
	}
	if ( x <= 0.0 ) {
		return 0.0;
	}
	if ( x >= 1.0 ) {
		return 1.0;
	}
	return log1p( c * x ) / log1p( c );
}
Returns the cumulative distribution function (CDF) for a Bradford distribution with shape parameter `c` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} c - shape parameter
@returns {Probability} evaluated CDF

@example
var v = cdf( 0.1, 0.1 );
// returns ~0.104

@example
var v = cdf( 0.5, 5.0 );
// returns ~0.699

@example
var v = cdf( 1.0, 10.0 );
// returns 1.0

@example
var v = cdf( -1.0, 0.5 );
// returns 0.0

@example
var v = cdf( 2.0, 0.5 );
// returns 1.0

@example
var v = cdf( 0.5, 0.0 );
// returns NaN

@example
var v = cdf( 0.5, -5.0 );
// returns NaN

@example
var v = cdf( NaN, 1.0 );
// returns NaN

@example
var v = cdf( 1.0, NaN );
// returns NaN