code: public static BitfinexInstrument build(final String symbolString) {
  		
  		if(symbolString.startsWith("t")) {
  			return BitfinexCurrencyPair.fromSymbolString(symbolString);
  		} else if(symbolString.startsWith("f")) {
  			return BitfinexFundingCurrency.fromSymbolString(symbolString);
  		} else {
  			throw new IllegalArgumentException("Unable to build currency for symbol string: " + symbolString);
  		}
  	}
docs: Build the bitfinex currency from sybol string
  @param symbolString
  @return