function logpdf( x, k, lambda ) {
	var xol;
	if (
		isnan( k ) ||
		isnan( lambda ) ||
		k <= 0.0 ||
		lambda <= 0.0
	) {
		return NaN;
	}
	if ( x < 0.0 ) {
		return NINF;
	}
	if ( x === PINF || x === NINF ) {
		return NINF;
	}
	if ( x === 0.0 ) {
		return ( k === 1.0 ) ? ln( k/lambda ): NINF;
	}
	xol = x / lambda;
	return ln( k / lambda ) + ( ( k - 1.0 ) * ln( xol ) ) - pow( xol, k );
}
Evaluates the natural logarithm of the probability density function (PDF) 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} evaluated logarithm of probability density function

@example
var y = logpdf( 2.0, 1.0, 0.5 );
// returns ~-3.307

@example
var y = logpdf( 0.1, 1.0, 1.0 );
// returns ~-0.1

@example
var y = logpdf( -1.0, 4.0, 2.0 );
// returns -Infinity

@example
var y = logpdf( NaN, 0.6, 1.0 );
// returns NaN

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

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

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