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

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

@example
var y = quantile( 0.8, 2.0, 1.0 );
// returns ~0.894

@example
var y = quantile( 0.5, 4.0, 2.0 );
// returns ~0.686

@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( 0.5, NaN, 1.0 );
// returns NaN

@example
var y = quantile( 0.5, 1.0, NaN );
// returns NaN

@example
var y = quantile( 0.5, -1.0, 1.0 );
// returns NaN

@example
var y = quantile( 0.5, 1.0, -1.0 );
// returns NaN