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

	/**
	* Evaluates the moment-generating function (MGF) for a Planck distribution.
	*
	* @private
	* @param {number} t - input value
	* @returns {number} evaluated MGF
	*
	* @example
	* var y = mgf( 0.5 );
	* // returns <number>
	*/
	function mgf( t ) {
		if ( isnan( t ) ) {
			return NaN;
		}
		return expm1( -lambda ) / expm1( t - lambda );
	}
}
Returns a function for evaluating the moment-generating function (MGF) of a Planck distribution with shape parameter `lambda`.

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

@example
var mgf = factory( 0.8 );
var y = mgf( -0.2 );
// returns ~0.8711