code: public static BitfinexCurrencyPair of(final String currency, final String profitCurrency) {
  		final String key = buildCacheKey(currency, profitCurrency);
  
  		final BitfinexCurrencyPair bcp = instances.get(key);
  
  		if (bcp == null) {
  			throw new IllegalArgumentException("CurrencyPair is not registered: " + currency + " " + profitCurrency);
  		}
  
  		return bcp;
  	}
docs: Retrieves bitfinex currency pair
  
  @param currency       currency (from)
  @param profitCurrency currency (to)
  @return BitfinexCurrencyPair