private String readElement(List<String> operators, List<String> operands, String s, int i) {
  		String s2 = s.substring(i).trim(), ret = "";

	    for (String tmp: operands) {
	    	if (s2.startsWith(tmp) && tmp.length() > ret.length()) {
	    		ret = tmp;
	    	}
	    }
	    if (ret.equals("")) {
	    	for (String tmp: operators ) {
	    		if (s2.startsWith(tmp)) {
	    			ret = tmp;
	    			break;
	    		}
	    	}
	    }
	    if (ret.equals("")) {
	    	return null;
	    }
	    return ret;
  	}