code: private function addRate($pair, $rate)
      {
          if (!preg_match('~^[A-Z]{3}/[A-Z]{3}$~', $pair)) {
              throw new InvalidArgumentException("Unexpected currency pair '$pair'.");
          }
  
          if (!(is_float($rate) && 0 < $rate)) {
              throw new InvalidArgumentException("Unexpected rate '$rate'.");
          }
  
          $this->rates[$pair] = $rate;
  
          return $this;
      }
docs: Adds the conversion rate.
  
  @param string $pair
  @param float  $rate
  
  @return ArrayCurrencyConverter