function pmf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( isNonNegativeInteger( x ) ) {
			return -expm1( -lambda ) * exp( -lambda * x );
		}
		return 0.0;
	}
Evaluates the probability mass function (PMF) for a Planck distribution.

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

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