function logpmf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( isNonNegativeInteger( x ) ) {
			return ln( -expm1( -lambda ) ) - ( lambda * x );
		}
		return NINF;
	}
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>