function logpdf( x, lambda ) {
	if (
		isnan( x ) ||
		isnan( lambda ) ||
		lambda < 0.0 ||
		lambda === PINF
	) {
		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 with rate parameter `lambda` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} lambda - rate parameter
@returns {number} evaluated logPDF

@example
var y = logpdf( 0.3, 4.0 );
// returns ~0.186

@example
var y = logpdf( 2.0, 0.7 );
// returns ~-1.757

@example
var y = logpdf( -1.0, 0.5 );
// returns -Infinity

@example
var y = logpdf( 0, NaN );
// returns NaN

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

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