function pmf( x, lambda ) {
	var lnl;
	if ( isnan( x ) || isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	if ( lambda === 0.0 ) {
		return ( x === 0.0 ) ? 1.0 : 0.0;
	}
	if ( isNonNegativeInteger( x ) && x !== PINF ) {
		lnl = (x * ln( lambda )) - lambda - factorialln( x );
		return exp( lnl );
	}
	return 0.0;
}
Evaluates 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 {Probability} evaluated PMF

@example
var y = pmf( 4.0, 3.0 );
// returns ~0.168

@example
var y = pmf( 1.0, 3.0 );
// returns ~0.149

@example
var y = pmf( -1.0, 2.0 );
// returns 0.0

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

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

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