code: boolean precedes(final String s1, final String s2) {
      String symbol1 = s1.toLowerCase(Locale.ENGLISH);
      String symbol2 = s2.toLowerCase(Locale.ENGLISH);
  
      if (!precedenceMap.keySet().contains(symbol1)) {
        return false;
      }
  
      if (!precedenceMap.keySet().contains(symbol2)) {
        return false;
      }
  
      int index1 = ((Integer) precedenceMap.get(symbol1)).intValue();
      int index2 = ((Integer) precedenceMap.get(symbol2)).intValue();
  
      boolean precedesResult = (index1 < index2);
  
      return precedesResult;
    }
docs: Determines whether one symbol precedes another.
  @param s1 symbol 1
  @param s2 symbol 2
  @return true if symbol 1 precedes symbol 2