function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 ) {
			return NINF;
		}
		return -( x*lambda ) + ln( lambda );
	}
Evaluates the natural logarithm of the probability density function (PDF) for an exponential distribution.

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

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