function factory( lambda ) {
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return constantFunction( NaN );
	}
	return logpmf;

	/**
	* Evaluates the logarithm of the probability mass function (PMF) for a Planck distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {NonPositiveNumber} evaluated logPMF
	*
	* @example
	* var y = logpmf( 2.0 );
	* // returns <number>
	*/
	function logpmf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( isNonNegativeInteger( x ) ) {
			return ln( -expm1( -lambda ) ) - ( lambda * x );
		}
		return NINF;
	}
}
Returns a function for evaluating the logarithm of the probability mass function (PMF) for a Planck distribution with shape parameter `lambda`.

@param {PositiveNumber} lambda - shape parameter
@returns {Function} logPMF

@example
var logpmf = factory( 0.5 );
var y = logpmf( 3.0 );
// returns ~-2.4328

y = logpmf( 1.0 );
// returns ~-1.4328