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

	/**
	* Evaluates the moment-generating function (MGF) for a Laplace (double exponential) distribution.
	*
	* @private
	* @param {number} t - input value
	* @returns {number} evaluated MGF
	*
	* @example
	* var y = mgf( 0.5 );
	* // returns <number>
	*/
	function mgf( t ) {
		var bt;
		if ( abs( t ) >= 1.0/b ) {
			return NaN;
		}
		bt = b * t;
		return exp( mu * t ) / ( 1.0 - pow( bt, 2.0 ) );
	}
}
Returns a function for evaluating the moment-generating function (MGF) of a Laplace (double exponential) distribution with location parameter `mu` and scale parameter `b`.

@param {number} mu - mean
@param {NonNegativeNumber} b - scale parameter
@returns {Function} MGF

@example
var mgf = factory( 4.0, 2.0 );

var y = mgf( 0.2 );
// returns ~2.649

y = mgf( 0.4 );
// returns ~13.758