code: public String getRequiredSymbol(String key) {
          String result = null;
          Object symbol = getSymbol(key);
          if (symbol == null) {
              throw new FitFailureException("No Symbol defined with key: " + key);
          } else {
              result = symbol.toString();
          }
          return result;
      }
docs: Gets symbol value, or throws exception if no symbol by that key exists.
  @param key symbol's key.
  @return symbol's value.