function logcdf( x, k, lambda ) {
	var p;
	if (
		isnan( k ) ||
		isnan( lambda ) ||
		k <= 0.0 ||
		lambda <= 0.0
	) {
		return NaN;
	}
	if ( x < 0.0 ) {
		return NINF;
	}
	p = -pow( x / lambda, k );
	return ( p < LNHALF ) ? log1p( -exp( p ) ) : ln( -expm1( p ) );
}
Evaluates the natural logarithm of the cumulative distribution function (CDF) for a Weibull distribution with shape parameter `k` and scale parameter `lambda` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} k - shape parameter
@param {PositiveNumber} lambda - scale parameter
@returns {number} natural logarithm of CDF

@example
var y = logcdf( 2.0, 1.0, 1.0 );
// returns ~-0.145

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

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

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

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

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

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

@example
var y = logcdf( 2.0, 0.0, -1.0 );
// returns NaN