function mean( alpha, beta ) {
	if ( alpha <= 1.0 || beta <= 0.0 ) {
		return NaN;
	}
	return beta / ( alpha - 1.0 );
}
Returns the expected value of an inverse gamma distribution.

@param {PositiveNumber} alpha - shape parameter
@param {PositiveNumber} beta - rate parameter
@returns {PositiveNumber} expected value

@example
var v = mean( 4.0, 12.0 );
// returns 4.0

@example
var v = mean( 8.0, 2.0 );
// returns ~0.286

@example
var v = mean( 1.0, 1.0 );
// returns NaN

@example
var v = mean( 1.0, -0.1 );
// returns NaN

@example
var v = mean( -0.1, 1.0 );
// returns NaN

@example
var v = mean( 2.0, NaN );
// returns NaN

@example
var v = mean( NaN, 2.0 );
// returns NaN