function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 || x === PINF ) {
			return NINF;
		}
		return ln( gammaDeriv( alpha, x * beta ) ) + ln( beta );
	}
Evaluates the logarithm of the probability density function (PDF) for a gamma distribution.

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

@example
var y = logpdf( -1.2 );
// returns <number>