function quantile( p, v ) {
	var prob;
	var xs;
	if (
		isnan( v ) ||
		isnan( p ) ||
		v <= 0.0 ||
		p < 0.0 ||
		p > 1.0
	) {
		return NaN;
	}
	prob = ( p > 0.5 ) ? 1.0 - p : p;
	xs = kernelBetaincinv( v / 2.0, 0.5, 2.0 * prob, 1.0 - (2.0 * prob) );
	return sign( p - 0.5 ) * sqrt( v * xs[ 1 ] / xs[ 0 ] );
}
Evaluates the quantile function for a Student's t distribution with degrees of freedom `v` at a probability `p`.

@param {Probability} p - input value
@param {PositiveNumber} v - degrees of freedom
@returns {number} evaluated quantile function

@example
var y = quantile( 0.8, 1.0 );
// returns ~1.376

@example
var y = quantile( 0.1, 1.0 );
// returns ~-3.078

@example
var y = quantile( 0.5, 0.1 );
// returns 0.0

@example
var y = quantile( -0.2, 0.1 );
// returns NaN

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

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

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