function quantile( p, lambda ) {
	if (
		isnan( lambda ) ||
		lambda < 0.0 ||
		lambda === PINF ||
		isnan( p ) ||
		p < 0.0 ||
		p > 1.0
	) {
		return NaN;
	}
	return -ln( 1.0 - p ) / lambda;
}
Evaluates the quantile function for an exponential distribution with rate parameter `lambda` at a probability `p`.

@param {Probability} p - input value
@param {PositiveNumber} lambda - rate parameter
@returns {number} evaluated quantile function

@example
var y = quantile( 0.8, 1.0 );
// returns ~1.609

@example
var y = quantile( 0.5, 4.0 );
// returns ~0.173

@example
var y = quantile( 0.5, 0.1 );
// returns ~6.931

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