code: function quantile( p, lambda ) {
  	if ( p !== p || p < 0 || p > 1 ) {
  		return NaN;
  	}
  	return - ln( 1 - p ) / lambda;
  }
docs: QUANTILE // 
  FUNCTION: quantile( p, lambda )
  Evaluates the quantile function for an exponential distribution with rate parameter `lambda` at a probability `p`.
  
  @param {Number} p - input value
  @param {Number} lambda - rate parameter
  @returns {Number} evaluated quantile function