code: public static BitfinexCurrencyPair register(final String currency, 
  			final String profitCurrency, final double minimalOrderSize) {
  
  		final String key = buildCacheKey(currency, profitCurrency);
  
  		final BitfinexCurrencyPair newCurrency = new BitfinexCurrencyPair(currency, profitCurrency, 
  				minimalOrderSize);
  
  		final BitfinexCurrencyPair oldCurrency = instances.putIfAbsent(key, newCurrency);
  
  		// The currency was already registered
  		if(oldCurrency != null) {
  			throw new IllegalArgumentException("The currency " + key + " is already known");
  		}
  
  		return newCurrency;
  	}
docs: Registers currency pair for use within library
  
  @param currency         currency (from)
  @param profitCurrency   currency (to)
  @param minimalOrderSize minimal order size
  @return registered instance of {@link BitfinexCurrencyPair}