function logpmf( x, lambda ) {
	if ( isnan( x ) || isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	if ( lambda === 0.0 ) {
		return ( x === 0.0 ) ? 0.0 : NINF;
	}
	if ( isNonNegativeInteger( x ) && x !== PINF ) {
		return ( x * ln( lambda ) ) - lambda - factorialln( x );
	}
	return NINF;
}
Evaluates the natural logarithm of the probability mass function (PMF) for a Poisson distribution with mean parameter `lambda` at a value `x`.

@param {number} x - input value
@param {NonNegativeNumber} lambda - mean parameter
@returns {number} evaluated logPMF

@example
var y = logpmf( 4.0, 3.0 );
// returns ~-1.784

@example
var y = logpmf( 1.0, 3.0 );
// returns ~-1.901

@example
var y = logpmf( -1.0, 2.0 );
// returns -Infinity

@example
var y = logpmf( 0.0, NaN );
// returns NaN

@example
var y = logpmf( NaN, 0.5 );
// returns NaN

@example
// Invalid mean parameter:
var y = logpmf( 2.0, -0.5 );
// returns NaN