function pmf( x, lambda ) {
	if ( isnan( x ) || isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	if ( isNonNegativeInteger( x ) ) {
		return -expm1( -lambda ) * exp( -lambda * x );
	}
	return 0.0;
}
Evaluates the probability mass function (PMF) for a Planck distribution with shape parameter `lambda` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} lambda - shape parameter
@returns {Probability} evaluated PMF

@example
var y = pmf( 4.0, 0.3 );
// returns ~0.0781

@example
var y = pmf( 2.0, 1.7 );
// returns ~0.0273

@example
var y = pmf( -1.0, 2.5 );
// returns 0.0

@example
var y = pmf( 0.0, NaN );
// returns NaN

@example
var y = pmf( NaN, 0.5 );
// returns NaN

@example
// Invalid shape parameter:
var y = pmf( 2.0, -1.0 );
// returns NaN