function pmf( x ) {
		var lnl;
		if ( isnan( x ) ) {
			return NaN;
		}
		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.

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

@example
var y = pmf( 2.0 );
// returns <number>