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 );
	}
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>