code: public static function EXPONDIST($value, $lambda, $cumulative)
      {
          $value = Functions::flattenSingleValue($value);
          $lambda = Functions::flattenSingleValue($lambda);
          $cumulative = Functions::flattenSingleValue($cumulative);
  
          if ((is_numeric($value)) && (is_numeric($lambda))) {
              if (($value < 0) || ($lambda < 0)) {
                  return Functions::NAN();
              }
              if ((is_numeric($cumulative)) || (is_bool($cumulative))) {
                  if ($cumulative) {
                      return 1 - exp(0 - $value * $lambda);
                  }
  
                  return $lambda * exp(0 - $value * $lambda);
              }
          }
  
          return Functions::VALUE();
      }
docs: EXPONDIST.
  
  Returns the exponential distribution. Use EXPONDIST to model the time between events,
  such as how long an automated bank teller takes to deliver cash. For example, you can
  use EXPONDIST to determine the probability that the process takes at most 1 minute.
  
  @param float $value Value of the function
  @param float $lambda The parameter value
  @param bool $cumulative
  
  @return float