function mgf( t, lambda ) {
	if ( isnan( t ) || isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	return exp( lambda * ( exp(t) - 1.0 ) );
}
Evaluates the moment-generating function (MGF) for a Poisson distribution with mean parameter `lambda` at a value `t`.

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

@example
var y = mgf( 1.0, 1.5 );
// returns ~13.163

@example
var y = mgf( 0.5, 0.5 );
// returns ~1.383

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

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

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