function logcdf( x, a, b ) {
	if (
		isnan( x ) ||
		isnan( a ) ||
		isnan( b ) ||
		a <= 0.0 ||
		b <= 0.0
	) {
		return NaN;
	}
	if ( x <= 0.0 ) {
		return NINF;
	}
	if ( x >= 1.0 ) {
		return 0.0;
	}
	return ln( 1.0 - pow( 1.0 - pow( x, a ), b ) );
}
Evaluates the natural logarithm of the cumulative distribution function (CDF) for a Kumaraswamy's double bounded distribution with first shape parameter `a` and second shape parameter `b` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} a - first shape parameter
@param {PositiveNumber} b - second shape parameter
@returns {number} evaluated logCDF

@example
var y = logcdf( 0.5, 1.0, 1.0 );
// returns ~-0.693

@example
var y = logcdf( 0.5, 2.0, 4.0 );
// returns ~-0.38

@example
var y = logcdf( 0.2, 2.0, 2.0 );
// returns ~-2.546

@example
var y = logcdf( 0.8, 4.0, 4.0 );
// returns ~-0.13

@example
var y = logcdf( -0.5, 4.0, 2.0 );
// returns -Infinity

@example
var y = logcdf( 1.5, 4.0, 2.0 );
// returns 0.0

@example
var y = logcdf( 2.0, -1.0, 0.5 );
// returns NaN

@example
var y = logcdf( 2.0, 0.5, -1.0 );
// returns NaN

@example
var y = logcdf( NaN, 1.0, 1.0 );
// returns NaN

@example
var y = logcdf( 0.0, NaN, 1.0 );
// returns NaN

@example
var y = logcdf( 0.0, 1.0, NaN );
// returns NaN