function quantile( p, alpha, beta ) {
	if (
		isnan( alpha ) ||
		isnan( beta ) ||
		isnan( p ) ||
		alpha < 0.0 ||
		beta <= 0.0 ||
		p < 0.0 ||
		p > 1.0
	) {
		return NaN;
	}
	if ( alpha === 0.0 ) {
		return 0.0;
	}
	return ( 1.0 / beta ) * gammaincinv( p, alpha );
}
Evaluates the quantile function for a gamma distribution with shape parameter `alpha` and rate parameter `beta` at a probability `p`.

@param {Probability} p - input value
@param {NonNegativeNumber} alpha - shape parameter
@param {PositiveNumber} beta - rate parameter
@returns {number} evaluated quantile function

@example
var y = quantile( 0.8, 2.0, 1.0 );
// returns ~2.994

@example
var y = quantile( 0.5, 4.0, 2.0 );
// returns ~1.836

@example
var y = quantile( 1.1, 1.0, 1.0 );
// returns NaN

@example
var y = quantile( -0.2, 1.0, 1.0 );
// returns NaN

@example
var y = quantile( NaN, 1.0, 1.0 );
// returns NaN

@example
var y = quantile( 0.0, NaN, 1.0 );
// returns NaN

@example
var y = quantile( 0.0, 1.0, NaN );
// returns NaN

@example
// Non-positive shape parameter:
var y = quantile( 0.5, -1.0, 1.0 );
// returns NaN

@example
// Non-positive rate parameter:
var y = quantile( 0.5, 1.0, -1.0 );
// returns NaN