function quantile( p, sigma ) {
	var s2;
	if ( isnan( sigma ) || sigma < 0.0 ) {
		return NaN;
	}
	if ( isnan( p ) || p < 0.0 || p > 1.0 ) {
		return NaN;
	}
	if ( sigma === 0.0 ) {
		return 0.0;
	}
	s2 = sigma * sigma;
	return sqrt( -2.0 * s2 * log1p( -p ) );
}
Evaluates the quantile function for a Rayleigh distribution with scale parameter `sigma` at a probability `p`.

@param {Probability} p - input value
@param {NonNegativeNumber} sigma - scale parameter
@returns {number} evaluated quantile function

@example
var y = quantile( 0.8, 1.0 );
// returns ~1.794

@example
var y = quantile( 0.5, 4.0 );
// returns ~4.71

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

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

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

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

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