function mgf( t, lambda ) {
	if ( isnan( t ) || isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	return expm1( -lambda ) / expm1( t - lambda );
}
Evaluates the moment-generating function (MGF) for a Planck distribution with shape parameter `lambda` at a value `t`.

@param {number} t - input value
@param {PositiveNumber} lambda - shape parameter
@returns {number} evaluated MGF

@example
var y = mgf( 0.2, 0.5 );
// returns ~1.5181

@example
var y = mgf( 0.4, 1.5 );
// returns ~1.1645

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

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

@example
var y = mgf( 2.0, -1.0 );
// returns NaN