function logpmf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		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.

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

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