function logpdf( x ) {
		var xol;
		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 lnkl + ( ( k - 1.0 ) * ln( xol ) ) - pow( xol, k );
	}
Evaluates the natural logarithm of the probability density function (PDF) for a Weibull distribution.

@private
@param {number} x - input value
@returns {number} evaluated logPDF

@example
var y = logpdf( 2.3 );
// returns <number>