function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x <= 0.0 ) {
			return NINF;
		}
		return A - ln( x ) + ( B * pow( ln(x) - mu, 2.0 ) );
	}
Evaluates the natural logarithm of the probability density function (PDF) for a lognormal distribution.

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

@example
var y = logpdf( 2.5 );
// returns <number>